Heat a large, heavy pot or Dutch oven over medium heat and cook the bacon until crisp, turning as needed, 5 to 6 minutes in all. Remove to a plate lined with paper towels.
Pour off all but a tablespoon of the fat from the pan, and return to medium heat. Add the onion and sauté for 5 minutes, until golden brown. Add the cabbage and toss occasionally in the pot for 3 or 4 minutes until the cabbage begins to wilt and is well mixed with the onions.
Pour in the beer and bring to a simmer over medium-high heat. Partially cover the pot, lower the heat to medium-low, and simmer for about 30 minutes, stirring occasionally, until the cabbage is tender and most of the liquid is evaporated.
Crumble the reserved bacon and stir it into the cabbage. Serve hot or warm.
Notes
This would go very well with corned beef, but also with roast chicken, salmon, a steak, pretty much anything. The whiter the cabbage is, the sweeter it is — you can also use savoy cabbage instead of regular; either is fine. Keep the cabbage in the fridge until you are ready to use it.