Preheat the oven to 375°F (190°C).
Place the raw chicken and chopped mushrooms in a single layer on a baking sheet or casserole dish. Drizzle with 1 Tbsp oil and 2 Tbsp lemon juice, then sprinkle with 1/2 tsp salt, pepper to taste, 2 tsp onion powder, and 1 tsp garlic powder; toss to coat evenly.
Roast the chicken and mushrooms in the preheated oven for 30–35 minutes, or until the chicken reaches an internal temperature of 165°F (74°C).
Remove from the oven and let cool slightly, then chop the chicken into bite-sized pieces and set the chicken and roasted mushrooms aside.
Increase the oven temperature to 425°F (220°C). Roll out the 1 pound of gluten-free pizza dough on a lightly floured surface and transfer to a 12-inch pizza tray.
Blind-bake (pre-bake) the crust for 10–15 minutes, until it feels slightly firm and begins to brown.
Spread 1/2 cup pesto sauce evenly over the par-baked crust, then arrange the baby spinach leaves on top of the pesto.
Top with the grated mozzarella, then distribute the chopped roasted chicken and mushrooms over the cheese.
Bake the pizza at 425°F (220°C) for 10–15 minutes, until the crust is golden-brown and the cheese is melted and lightly browned.
If desired for extra browning, broil on high for 2–3 minutes—watch closely to prevent burning.