Pizza Stuffed Mushroom Burgers

16 oz. (1 pound) bulk Italian sausage - plant-based or traditional

1/2 medium red onion, diced

1 bell pepper (any color), diced

3/4 cup sliced black olives

1/2 tsp. Italian seasoning

1/2 tsp. kosher salt

1/4 tsp. ground black pepper

4 portobello mushrooms

~1 Tbsp. extra-virgin olive oil

Kosher salt and ground black pepper, to taste

3/4 cup marinara sauce

4 slices provolone cheese (or mozzarella)

4 hamburger buns of your choice

Baby spinach leaves, for serving 

Pesto Mayo: 

1/4 cup prepared basil pesto 

1/2 cup mayonnaise 

1. Start by making the filling for the stuffed mushrooms. In a large skillet over medium heat, cook the Italian sausage until golden brown and cooked through. 
2. Once the sausage is browned, add the diced onion and bell pepper, and cook just until beginning to soften, about 3-5 minutes. Remove the skillet from the heat, and add the sliced black olives, Italian seasoning, salt, and pepper. Set aside. 
3. Prepare the portobello mushrooms by wiping the tops of any dirt or debris with a damp cloth. Remove the stems. Using a spoon, gently scrape out the gills of the mushrooms, being careful not to tear the edge of the cap. Brush each portobello mushroom with olive oil, and season lightly with salt and pepper. 
4. Heat a grill pan over medium heat until warm. Place mushrooms, bottom side down, on the grill. Cook until just beginning to soften around the edges, about 5 minutes. 
5. Flip the mushrooms so they now resemble cups. Place several spoonfuls of the reserved sausage mixture into each mushroom (load ‘em up!). Next, top each sausage-filled mushroom with about 3 Tbsp. of marinara sauce. Finally, place a slice of provolone cheese over the top of each stuffed mushroom. Cover the grill with a large heat-safe lid to trap steam and help melt the cheese. Cook about 3-5 more minutes, until cheese is melted. 
6. Remove the stuffed mushrooms from the grill. Place each mushroom on the bottom of a bun that has been prepared with a smear of Pesto Mayo and a handful of baby spinach leaves, then place the other bun half on top. Serve, and enjoy!