With a sharp knife or kitchen fork, prick each sausage several times. In a saucepan, combine the red onions and beer and bring to a simmer over medium heat. Add the sausages and poach until almost cooked through, about 5 minutes. Remove the sausages from the liquid and set aside, reserving the liquid.

In a large pot, cook the bacon over medium heat until crisp, about 5 minutes. Remove with a slotted spoon and drain on paper towels. Add the yellow onions to the fat in the pan and cook, stirring, until very soft, 5 to 6 minutes. Add garlic and cook, stirring, for 30 seconds. Add the apples and cook, stirring, until starting to soften, about 2 minutes. Add the bay leaf, peppercorns, and juniper berries and cook, stirring, for 30 seconds. Add the sauerkraut and reserved beer cooking liquid (without the onions), sugar, salt and pepper, and bring to a boil. Reduce the heat and simmer uncovered until tender and aromatic, stirring occasionally, about 1 1/2 hours.

Preheat the grill. Grill the sausages until cooked through, about 5 minutes, turning to cook evenly on all sides. Remove and place on a platter.

Remove the sauerkraut from the heat and discard the bay leaf. Stir in the cooked bacon and adjust the seasoning, to taste. Arrange the sauerkraut on the platter with the sausages and serve, if desired, with rolls and mustard. Serve with cold beer.