1. Preheat the oven to 250 degrees F. Line a baking sheet with parchment paper. Cut one lemon into 1/8-inch-thick slices, and juice the second lemon. Arrange the slices on the prepared baking sheet and place the thyme sprigs on top. Add the salmon fillets, skin side down. Drizzle the lemon juice and oil over the salmon, then sprinkle with the salt and pepper. Place in the oven on the center rack and roast until the salmon is cooked almost all through at the thickest point, about 25 minutes. To check for doneness, make an incision with a sharp knife at the thickest point of a fillet and pull it apart to check for opaqueness; don't cook it beyond medium-rare.

2. To serve, spoon 2 to 3 tablespoons of the cauliflower onto each plate, then gently place a fillet on top. Garnish with the lemon slices and the thyme sprigs, or just the leaves. Serve immediately.