Preheat the oven to 400°F (200°C).
Steam or boil the asparagus for about 2 minutes, until it is half cooked. Plunge into ice water to stop the cooking, drain, and pat dry. Brush the onion pieces with a little olive oil and grill or roast in the oven until almost cooked through. Ten minutes on the grill or half an hour in the oven should do it.
Place the asparagus in a single layer in a roasting dish that can be taken to the table. Lay the salmon fillets diagonally across the asparagus. Tuck the tomatoes and zucchini around the fish. Sprinkle the corn kernels in between the fillets and pour in enough stock to just cover the asparagus. Salt and pepper to taste. Cover with foil and cook for 12 to 15 minutes. If you like your salmon rare or are using wild salmon, decrease the cooking time by about 4 minutes.
Take off the foil, check the fish and cook until done, approximately 5 minutes. Remove the dish from the oven and drizzle each piece of salmon with ½ tsp. (2.5 mL) oil and a sprig of chervil.
Bring the roasting dish to the table and let guests help themselves, or, using large-rimmed soup bowls, place some asparagus in the bottom of each bowl, put a piece of salmon on top, and divide the vegetables among the six bowls. Pour the liquid down the side of each bowl and serve with thick slices of baguette to dunk in the flavored broth.