Preheat oven to 500 degrees F.
Heat a large skillet over medium high heat. Add oil and mushroom caps and season caps with salt and pepper. Saute caps 5 to 7 minutes, until they are lightly browned and tender on edges. Turn caps up and let juices drain away from caps. Transfer caps to a small nonstick baking sheet. Wipe out skillet and return to heat. Add a touch of oil and sausage to the hot skillet. Brown and crumble sausage for 3 minutes. In a food processor pulse and chop the garlic, add mushroom stems and pulse to chop the mushroom stems. Add celery, onion and red bell pepper to the mushrooms and pulse to chop. Remove mixture from the processor and saute veggies and mushrooms over medium high heat another 3 to 5 minutes. Add dry, defrosted spinach and stir into stuffing. Add chopped bread and cheese to the pan and toss stuffing until bread is moist and stuffing is combined, 2 or 3 minutes. Fill caps with stuffing using a small scoop or large spoon. Place caps in hot oven and reduce heat to 450 degrees F. Bake 6 to 8 minutes to crisp edges of stuffing and set stuffing in mushrooms. Transfer stuffed mushrooms to a serving plate.