Preheat oven to 180C fan-forced (200C conventional).
Mix the crunchy topping ingredients in a small bowl. Set aside.
For the filling: heat one tablespoon of the olive oil in a nonstick pan over medium-high heat. Add spinach and cook until wilted. Working in batches, squeeze excess residual water out of the spinach using a wooden spoon against the side of the pan. Place the spinach in a mixing bowl. Add spring onion, garlic, salt, pepper and ricotta and mix until combined.
Prepare the mushrooms: Trim off the stalks (reserve to toss into your next stir-fry), then cut a 2cm X incision in the cap, all the way through (see note). Rub the top of the mushroom caps with the remaining tablespoon of olive oil.
Stuff the mushrooms with the filling. Top with colby, followed by the crunchy topping.
Place the mushrooms on a rack set on a baking tray (see note). Bake for 30 minutes. Rest 5 minutes before serving.
