1 teaspoon olive oil, or more if needed
1 clove garlic, minced
4 large sweet onions (such as Vidalia(r)), thinly sliced
1 1/2 tablespoons finely chopped green bell pepper (optional)
1 1/2 tablespoons finely chopped red bell pepper (optional)
1/2 cup sliced fresh mushrooms (optional)
salt and black pepper to taste
1/4 cup butter
1 pound calf’s liver, skinned, deveined, and sliced
1/4 cup all-purpose flour
2 cups cold water
2 teaspoons beef bouillon granules
1 tablespoon red wine
Heat the olive oil in a large skillet over medium heat. Stir in the garlic, onion, green bell pepper, red bell pepper, and mushrooms; cook and stir until the onion has softened and turned translucent, about 5 minutes. Season with salt and pepper. Push onion mixture to the side of the skillet, and place the butter in the center of the skillet. Add calf’s liver, one slice at a time. Cover and cook until lightly browned, flipping liver slices once, 5 to 6 minutes. Cut liver slices in half.
Whisk together flour, water, and beef bouillon in a bowl until combined. Pour mixture into the skillet; cook and stir until gravy thickens, then add red wine. Cook for 2 more minutes. Remove from heat and serve.