Preheat the broiler. Pierce the sausages all over with a fork, arrange on a baking sheet and broil, turning halfway through, until golden brown, about 8 minutes (they will not be fully cooked). Transfer to a cutting board and cut in half on the diagonal.
Meanwhile, heat the olive oil in a large skillet over medium-high heat. Add the bell peppers, fennel and onion and cook, stirring, until they soften slightly, 3 minutes. Add 1/2 teaspoon salt, the garlic, rosemary, red pepper flakes and fennel seeds. Cook, stirring, 1 more minute.
Stir in 1/2 cup water, the tomatoes and basil. Reduce the heat to medium; cover and cook until the vegetables are tender, 5 minutes. Uncover; add the sausages and cook through, 3 to 4 minutes. (Add a splash of water if the sauce gets too thick.) Top with more basil.