Wrap salmon tightly in plastic wrap and place in the freezer for 45 minutes.
Unwrap the fish and use a sharp knife to slice it crosswise into very thin slices. Arrange on a serving platter. Cover with plastic wrap and refrigerate for 30 minutes.
Meanwhile, place a fine-mesh sieve over a heatproof medium bowl. Heat oil in a medium skillet over medium heat. Carefully add capers (they will spatter). Fry, swirling gently, until the capers are golden-brown, 30 seconds to 1 minute. Drain in the sieve. Let cool 5 minutes. Whisk lemon juice and parsley into the oil.
Top the salmon with fennel, onion, salt, pepper, the capers and the dressing. Garnish with fennel fronds and lemon wedges, if desired.
Tip: Raw wild salmon may contain parasites but commercial freezing kills them. Buy it from a high-quality fish market and tell them how you plan to serve it, or look for salmon labeled "frozen at sea."
177 calories;14 g fat(2 g sat); 1 g fiber; 2 g carbohydrates; 13 g protein; 12 mcg folate; 36 mg cholesterol; 1 g sugars; 0 g added sugars; 257 IU vitamin A; 4 mg vitamin C; 12 mg calcium; 0 mg iron; 207 mg sodium; 263 mg potassium