Cook broccoli: Bring a small amount of water to a boil in a saucepan, salt generously. Add the broccoli rabe, cover and steam for 2-3 minutes, until just wilted. Remove the broccoli rabe from the pan and immediately run under cold water to cool.
Cook polenta: Bring water and salt to a boil in a 3-quart heavy saucepan, then add polenta in a thin stream, whisking constantly. Cook over moderate heat, whisking, 2 minutes. Reduce heat to low and simmer, covered, stirring occasionally for 45 minutes. Stir in butter, cream, and cheese and remove from heat.
While polenta cooks, heat the oil in a large sauté pan. Add the garlic and sauté until golden, about 2 minutes. Add the sausage and broccoli and sauté 2 minutes. Season to taste with salt and pepper and serve over polenta.