Make Your Mondays Bubble and Squeak With This Recipe

Name: Make Your Mondays Bubble and Squeak With This Recipe
Cuisine: European, British
Ingredients: 6 tablespoons unsalted butter or vegetable oil; 1/2 cup onion, finely chopped; 1 pound mashed potatoes, about 2 cups; 1 cup leftover vegetables (e.g., cabbage, kale, peas, cooked swede, cooked carrots, and/or cooked Brussels sprouts), finely chopped; Optional: bacon, leftover roast, bangers, and/or ham, chopped; Salt, to taste; Freshly ground black pepper, to taste; Optional: fried or poached eggs, for serving