Prepare Peppers: Preheat your oven to 375°F (190°C). Cut the tops off the bell peppers and remove the seeds and membranes. Place the peppers upright in a baking dish and bake the peppers unstuffed for 10 minutes.
OPTIONAL STEP: If you want to cook the peppers quicker, do this instead of baking the unstuffed peppers in the oven first. Fill a large pot with water and bring it to a boil over high heat. Place the peppers in the boiling water and let them cook for 3-4 minutes, or until slightly softening. Use tongs to remove the peppers from the boiling water and place them on a plate lined with paper towels to drain any excess water. Add to a baking dish upright.
Slice the onion in thin slices, julienne the red pepper, and slice and clean the mushrooms. Slice the steak against the grain into thin strips.
In a large skillet, heat a drizzle of olive oil over medium-high heat. Add the thinly sliced steak to the skillet and cook until browned, about 2-3 minutes per side. Season with salt and pepper to taste. Remove the steak from the skillet and set aside.
In the same skillet add another 1 tbsp of olive oil to the hot skillet. Add in the minced garlic, mushrooms, red peppers, and onions. Cook until tender for 4-5 minutes. Add the steak with the veggie mixture along with a tablespoon of coconut aminos or Worcestershire sauce and cook for another minute or two.
To a small bowl add all the sauce ingredients and mix until combined.
Assemble each pepper by adding a big heaping spoonful of the veggie and meat mixture into each pepper. Once each pepper is filled, top with 2 tbsp of Mac's Awesome Sauce OR cheese of choice to each filled pepper.
Place the stuffed peppers in the oven and bake for 15-20 minutes, or until the cheese is melted and bubbly, and the peppers are tender.
Remove and garnish with chopped parsley.