Fry the bacon in a large pot until crispy. Add the onions and cook, stirring, until they are soft and lightly golden, 6 to 7 minutes. Add the salt, black pepper, and cayenne. Add the shallots and garlic and cook for 2 minutes. Stir in the beer, vinegar, and molasses. Add the greens, a third at a time, pressing them down in the pot as they wilt. Cook, uncovered, stirring occasionally, for about 1 hour and 15 minutes.