Start by selecting the high sauté setting on your Quickpot. Add the butter. Once melted, add the onions, sugar, salt, and whiskey. Stir with a wooden spoon, separating the onion layers and coating with fat.
Cover the pot and cook, stirring occasionally, until the onions turn a darker shade of brown, 4 to 6 minutes.
Stir in the flour and garlic. Cook until fragrant, about 2 minutes.
Pour in the Guinness and stir occasionally until reduced by one-third, about 5 minutes.
Stir in the broth, secure the lid, select the manual setting, and set the time for 5 minutes.
Preheat broiler. Divide soup into oven-safe bowls. Top each with baguette slices and cheddar cheese.
Broil until cheese is melted and bubbly.