Toss the pecans and pumpkin seeds into your food processor with garlic, mustard greens, parsley, and salt. Pulse until uniformly chopped and well combined.
Process on high speed while slowly drizzling the olive oil and lemon juice into the food processor until you've used it all. Transfer the pesto to a small bowl and enjoy right away, or store in an airtight container in the fridge for up to 1 week.