This is one of those times when a little context goes a long way. Historically cooks had far more generous ideas about what constituted herbs. Robert May shared his list of potential potherbs in a receipt for making pottage saying,

chopped amongst the herbs, as time [thyme], sweet marjoram, parsley, chives, salt, succory, marigold-leaves, and flowers, strawberry-leaves, violet-leaves, beets [leaves], borage, sorrel, blood-wort, sage, penny-royal

In fact, the difference between an herb and a spice is that herbs could be locally grown and obtained while spices were obtained from trade. The word species is Latin, meaning a commodity of special distinction or value, not an ordinary item of merchandise that was eventually anglicized to the word “spices.”

This receipt calls for ingredients that we more typically think of as herbs and I like the mix of beet greens (chard would work too) and spinach. 

TO MAKE A TART (TOURTE), take four handfuls of beets, two handfuls of parsley, a handful of chervil, a sprig of fennel and two handfuls of spinach, and pick them over and wash them in cold water, then cut them up very small; then bray with two sorts of cheese, to wit a hard and a medium, and then add eggs thereto, yolks and whites, and bray them in with the cheese; then put the herbs into the mortar and bray all together and also put therein some fine powder. Or instead of this have ready brayed in the mortar two heads of ginger and onto this bray your cheese, eggs and herbs and then cast old cheese scraped or grated onto the herbs and take it to the oven and then have your tart made and eat it hot.

The Goodman of Paris 1393

One thing to point out about my interpretation is that many ancient cultures grew beets but only ate the greens and that practice carried over for some time. Beetroot was mostly considered to be a medicinal laxative at first and eating them didn’t really become a common English practice until the Victorian era. I think it coincides with the popularity of Indian food. If an older receipt doesn’t specify using the root, I assume it means the leaf.

This is also an odd receipt in that it doesn’t specify blanching the ingredients in a direct manner which was pretty standard practice as you can see in the following receipts.

How to make Tartes of Spinage. Boyle your Spinage very tender, and three or foure apples with it, and when it is very tender, straine it through a faire cloth, and then season it with the yolk of an egge, Sugar, Sinamon, and Ginger.

A Book of Cookrye 1591

To make a Tarte of Spinadge. Take Spinadge and seeth it stalke and all, and when it is tenderly sodden, take it off, and let it drayne in a Cullynder. and then swing it in a clowte, and stampe it and straine it with two or three yolkes of egges, and then set it on a chafindish of coales, and season it with butter and Suger, and when the paste is hardened in the Ouen, put in this Comode, strake it euen.

The Good Housewife’s Jewell 1596

It is possible “pick them over” implied that sort of basic practice. Washing them in cold water is the second step in the blanching process. What I know for certain is that I made this without blanching the first time and far too much liquid cooks out of fresh greens for this to set up properly.

Ingredients
1 pound of greens – chopped and blanched
1/2 cup fresh parsley, chopped
1/4 c fresh chervil or chives
2 Tbsp chopped fresh fennel leaves
5 eggs
6 oz of hard cheese (parmesan cheese, white cheddar)
6 oz of soft cheese (mozzarella, swiss)
1/2 tsp ginger or galangal
1/2 tsp salt
9″ pie crust

Directions

1. Place your pie crust in a 9-inch pie pan.
2. Tear up greens and blanch them in boiling water. Lay them out on cheesecloth and roll them up in it to squeeze out the excess water.
3. Mince the fresh herbs finely.
4. Mix the greens and herbs in a mixing bowl with the cheeses. As indicated in the directions use one hard cheese and one softer cheese. It really does not matter what kind of cheese you use, as long as you stick to that.
5. Then blend the eggs up and pour them over this mixture.    
6. Stir it altogether well and pour it into the pie crust.
7. Bake at 350 degrees until set in the middle.  
8. Check it after 30 minutes.

References:
The Goodman of Paris Eds. G. G. Coulton and Eileen Power. Trans. Eileen Power. London: George Routledge & Sons, 1928 p. 278.