Preheat the oven to 450 degrees, and place a rimmed sheet pan in to heat.
In a mixing bowl, toss the tomatoes, bell peppers, green onion, and mushrooms with a good pinch of kosher salt and black pepper. Add a drizzle of extra virgin olive oil (2 to 3 tablespoons) and toss to make sure all the vegetables are well-coated.
Remove pan carefully, pour and spread vegetables evenly and return to oven. Cook vegetables for 10-15 minutes or until cooked and lightly charred.
Remove vegetables and turn oven heat to 400 degrees.
In your mixing bowl, whisk together the eggs, baking powder, milk, feta, parsley, thyme, and a good pinch of kosher salt and black pepper. Fold in the roasted vegetables and the spinach.
Coat the bottom and sides of a 10-inch cast iron or oven-safe pan and warm over medium-high heat until shimmering. Pour the egg and vegetable mixture in the pan and cook for 2 to 3 minutes to allow the bottom of the eggs to settle.
Transfer the pan to the heated oven and cook for 8 to 10 minutes or until the center of the eggs are cooked through and the center of the frittata is firm and no longer runny).
Serve with a garnish of fresh parsley.