Turn the nuts into meal in a food processor or a good blender (a few seconds).
Add lime juice (and garlic if using) and process until very creamy (you can add some of the olive oil if needed).
Add seasonings to taste and blend until well mixed.
Cut or twist off the ends of the stems of the parsley and cilantro then wash the herbs well (normally they take 3 soaks and lift-outs in a large bowl of water). Remove mint leaves (if using) from the stem and wash leaves if needed. Shake off water well.
Set aside a couple handfuls of herbs without their stems. Process the rest into the nuts until everything is green and no stems are left intact.
Adjust salt and other seasonings if needed. Add olive oil and blend in.
Add the remaining leaves and pulse the machine so that the leaves are all cut up well but still recognizable (or mince the leaves and mix them in by hand).
Wash the mushrooms and gently remove the stems and the fleshy part over the gills (chop what you've removed and put aside for a stir-fry another day).
Lightly oil a baking pan, place the mushrooms gill down on the pan to get some oil then flip to roast on their backs.
Bake until soft but still strong enough to pick up with your fingers. About 15 minutes at 350*F, but go by doneness, not time.
Dump any accumulated liquid and leave them upside down on the dry pan to cool.
Put a heaping spoonful of pesto in each mushroom.
Serve warm, room temperature, or cold. Store in single layers. They will last in the fridge for a day or two. Great for picnics.