1. Cook pasta according to package directions; drain.
2. Trim outer leaves from fennel. Chop fronds to measure 1/2 cup. Discard stalks. Cut bulb in half lengthwise; discard core. Thinly slice bulb.
3. Heat oil in a large skillet over medium-high heat. Add sliced fennel and onion; sauté 4 minutes. Add garlic; sauté 20 seconds. Stir in tomato sauce, oregano, red pepper, and sauce from sardines. Cover and reduce heat.
4. Discard backbones from fish. Add fish to pan; gently break fish into chunks. Cover and cook 6 minutes. Toss pasta with sauce; sprinkle with fronds.