Make the pesto. In a small food processor, combine the pepitas, garlic, and a few pinches of salt and pepper. Pulse until combined. Add the lemon juice, cilantro (or basil), and cumin and pulse again. With the blade running, drizzle in the olive oil and purée until smooth. Taste and adjust seasonings. Add the honey, if desired (it will reduce bitterness in the cilantro).
Make the sliders. Heat a grill or grill pan to medium heat.
Prepare the mushrooms by removing the stems and cleaning the caps with a damp cloth or paper towel. Place the mushrooms in a medium bowl and drizzle with olive oil, balsamic vinegar, tamari, and pepper. Toss until well coated. (I use my hands to rub it all over until they are fully coated on both sides.)
Place the mushrooms, gill side down, on the grill or grill pan. Cook about 10 minutes per side, or until the mushrooms are tender.
Place the mushrooms on the toasted buns and top each with a tomato slice, microgreens, and a generous slather of pepita pesto.