Place a Dutch oven on the stovetop over medium-high heat. Add olive oil and heat until shimmering. Add onions and sauté until beginning to brown, 4-6 minutes. Add bratwurst and cook until pink dissipates, 5-7 minutes. Add minced garlic and cook until fragrant, approximately 1 minute. Add dark beer and deglaze pan, then add the drained soaked black-eyed peas and greens. Cover with water, then add salt and pepper. Stir to combine. Bring pot to a rolling boil, then lower the heat and simmer uncovered until flavors meld and peas are tender, at least 30 minutes.