Irish Lassie's Liver and Onions

INGREDIENTS

• 1 teaspoon olive oil, or more if needed
• 1 clove garlic, minced
• 4 large sweet onions (such as Vidalia®), 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