Sprinkle both sides of thighs with salt and pepper. Heat a large non-stick skillet to medium-high and add 2 tablespoons of canola oil.
Add thighs to the skillet and cook about 4 minutes per side, or until each side is browned. Remove thighs and set on a plate.
Turn heat down to medium then add peppers and onions to the pan. Cook about 7 minutes or until softened (add more oil if pan is too dry).
Add garlic and cook 1 minute. Add diced tomatoes, tomato sauce, tomato paste, sugar, Italian seasoning, 1 teaspoon of salt, and pepper to taste. Stir to combine then add thighs back into pan. Reduce heat to low, cover and simmer for 20-25 minutes.
Just before serving, add parsley. Serve over short pasta or rice. Top individual servings with parmesan.
Notes
Sliced, fresh mushrooms can be added as well. Sauté with the peppers and onions before adding sauce ingredients.