Preheat the oven to 450 degrees
In a bowl large enough to marinate the mushroom, mix the olive oil, vinegar, garlic powder, thyme, salt, and pepper.
1 medium to large Portabello Mushroom, 1 teaspoon olive oil, 1 tablespoon balsamic vinegar, ¼ teaspoon garlic powder, ¼ teaspoon dried thyme, Pinch of kosher salt and pepper, ¼ of a large bell pepper chopped
Wash the mushroom and remove the stem. You can also peel a few pieces away from the cap. This will help the marinade penetrate a little easier.
Coat the mushroom in the marinade and let it sit while you prepare the rest of the ingredients.
Preheat a small skillet over medium-high heat and spray with non-stick cooking spray. Add the onion and let it scorch a little while cut the rest of the ingredients.
Non-stick Cooking Spray, 1 slice of sweet onion chopped
Chop the stem of the mushroom, pepper, and spinach. Add the mushroom and peppers next. Again, cooking until just scorched. Finally, add the chopped spinach and cook until it wilts.
½ cup baby spinach chopped
Remove the mushroom from the marinade and place upside down on a cookie sheet lined with aluminum foil.
Dice up the cheddar cheese and add it to the bowl with the marinade. Add the cooked the veggies from the skillet. Mix to make the filling.
¾ oz of cheddar cheese
Pile the filling onto the mushroom cap.
Bake for 10-12 minutes until the cheese is oozing and just starting to brown.