4 tablespoons unsalted butter
1 pound sweet onions, quartered and thinly sliced (about 4 cups)
1/2 pound red onions, quartered and thinly sliced (about 2 cups)
1/2 cup thinly sliced shallots
4 cloves garlic, minced
2 (15-ounce) cans reduced-sodium beef stock
1 tablespoon finely chopped fresh thyme or 1 teaspoon dried thyme
1/4 teaspoon salt, or to taste
1/4 teaspoon freshly ground pepper, or to taste

Melt the butter in a Dutch oven over medium heat. Add the onions. Stir well to coat with the butter. Reduce the heat to low. Cover and cook, stirring occasionally, until tender and golden brown, about 20 minutes. (Don’t let the onions burn or they will become bitter.)
Add the shallots and garlic. Stir constantly for about 2 minutes.
Stir in the beef stock and thyme. Increase the heat and bring to a boil. Reduce the heat to medium-low. Cover and simmer, stirring occasionally, for about 10 minutes. Stir in the salt and pepper. Taste and adjust the seasoning.

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