In a large skillet, heat the olive oil over medium-high heat. Add the sausage links and allow them to cook until browned on all sides, about 5 minutes total. Remove to a plate and set aside.
Add the onions to the skillet and cook until soft and slightly caramelized, 4 to 5 minutes. While the onions are cooking, cut the sausages into slices about 1/2-inch thick. Add the sausage and sage to the skillet and cook everything together, stirring occasionally, until the sausage is cooked through and crispy around the edges, 5 to 6 minutes.
Add the slices of apples and cook them only until they begin to soften but still retain their shape, 4 to 5 minutes. Season everything with a good pinch of salt and pepper and serve over Belgian waffles or pancakes with a good drizzle of warm maple syrup.
I usually use Granny Smith apples for this dish, but you can use any that you have locally in season. You can also try a mix of textures and flavors, tart and sweet, crispy and soft, from a local orchard or farmers' market: Believe me, it will be amazing!