Traditionally, seven kinds of herbs go into this:
melissa (this one is optional and some recipes don't even include it)
Depending on where you live, you might not be able to find all these herbs, but that's okay. Parsley, chives, and dill should work for a minimalist version, though I suggest you add some lemon juice to mimic the sorrel's acidity. You can also use frozen herbs in a pinch; obviously fresh herbs will give you the most aromatic result. Oh, I have no idea how much precisely you should use of each herb, I had about 150 g total, though if you use frozen, you'll want more to get enough flavor.
You will also need:
750 ml (25 oz) soy yogurt, unsweetened
1 - 2 tablespoons Dijon mustard
1 - 2 perfect avocados (or use tofu instead, about 1 - 2 cups, diced)
1 - 2 teaspoons soy sauce
salt and pepper to taste
Chop all the herbs finely. Chop your avocados or tofu and the other ingredients, mix well. Traditionally, this is eaten with boiled potatoes, as you can see I dared bake mine in the oven with some herbs, salt and pepper.