Boil a large pot of salted water, add tagliatelle pasta, and cook until al dente, about 8-10 minutes. Reserve a cup of pasta water before draining.

In a skillet, heat olive oil over medium heat. Add minced garlic and diced onions; sauté until the onions are translucent, about 3-4 minutes.

Add sliced baby bella mushrooms and zucchini to the skillet; sauté for 3-5 minutes until browned and tender.

Sprinkle in red pepper flakes, black pepper, and salt. Stir in pesto until well coated, warming through for another minute.

Mix drained pasta into the skillet, tossing to combine. Add reserved pasta water if needed to moisten the dish.

Serve hot, garnished with parsley and parmesan cheese if desired.