Torta d'Erbe is one of those things that doesn't have a recipe. It's a sort of baked vegetable pie made from gathered vegetables, mostly green, whatever is at hand.
Our Lunigiana neighbor Francesca's version is a simple version of torta d'erbe without a top crust. It uses steamed vegetables (in this case zucchini and onions), 4 eggs, grated Parmigiano-Reggiano cheese, ricotta, salt to taste, and a dusting of nutmeg. When the vegetables are cool, everything is mixed together well with a wooden spoon.
The first step is to put the chopped-up vegetables into a pan with just a little water and steam them. Drain them, and put in a bowl to cool to room temperature, then follow along with the pictures below.
Hopefully the pictures will give you an idea of the quantities of ingredients. Everything is quite variable; the eggs will hold it all together after it cooks.
Torta d'Erbe, like Italian food in general, is regional, and depends heavily on what's available (and what's always been available in your backyard). You can use any kind of vegetables you wish. Popular are leek and potato, or you can use a combination of field greens and herbs. A Bechamel sauce can be substituted for the ricotta.
Most versions of Torta d'erbe, especially those you see in markets and at festivals, have a pastry crust.
Torta d'erbe, like bread, is best baked in a forno di legno, a wood fired oven. Around here it's done that way on our local feast days, when our communal oven can be filled with pies and meats.