Preheat the oven to 375 degrees F (190 degrees C). Grease a 9x13-inch baking dish with 1 tablespoon butter.

Heat oil in a large skillet over medium-high heat. Sauté onion and garlic in hot oil until onion begins to soften, 5 to 7 minutes. Add bacon, bell pepper, and jalapeños; cook and stir until bacon is browned, 7 to 10 minutes.

Stir beer, barbecue sauce, brown sugar, and molasses together in a large bowl. Stir in pinto beans, bacon-onion mixture, garlic salt, and pepper. Pour bean mixture into the prepared baking dish.

Bake in the preheated oven until bubbling and browned, about 45 minutes.