Heat 2 tablespoons of oil in a medium saute pan or griddle over medium heat. Add the onions and cook until soft. Add the garlic and fennel and cook for 1 minute. Remove from the heat, stir in the mustard and sage, and let cool for 5 minutes. Place the pork in a bowl, add the onion mixture, and gently toss to combine. Season with salt and pepper.
Heat 2 tablespoons of oil in a large nonstick skillet over high heat. Form the sausage mixture into 8 equal size patties. Cook on both sides until golden brown. Reduce the heat, cover, and continue cooking until just cooked through. Serve warm.