Shallow-Poached Salmon with Leek Beurre Blanc

Preheat oven to 500°F. Sprinkle fillets evenly with salt and pepper. Fold a 12-inch square of parchment paper in half from bottom to top to create a rectangle; fold left side over to form a square. Fold bottom right corner up to top left corner to form a triangle. Set the bottom left point of the parchment triangle in the center of a 12-inch ovenproof skillet. Mark where the outer edge of the parchment meets the bottom edge of the skillet; trim parchment along the mark in an arc shape to mimic the curve of the skillet. Snip 1/2 inch from the folded point of the triangle. Unfold cartouche, and set aside.

Heat oil and 2 tablespoons butter in a 12-inch ovenproof skillet over medium-high until butter is melted. Add lemon slices; cook until lemon flesh is bubbling, about 1 minute. Flip lemon slices. Add leeks to skillet; stir to coat. Cook, stirring occasionally, until leeks are wilted, about 5 minutes. Add thyme sprigs to skillet; top with salmon fillets. Add wine to skillet, and top mixture with parchment cartouche. Bring to a simmer over medium-high. Carefully transfer skillet to preheated oven. Roast until fish is opaque and flaky, about 6 minutes.

Carefully remove skillet from oven. Remove cartouche; transfer fillets to a warm platter, and re-cover with cartouche. Return skillet to heat over medium; bring pan juices to a light simmer. Add tomatoes and remaining 1/4 cup butter; cook, stirring constantly, until butter is melted and sauce is creamy, about 2 minutes. If desired, add salt to taste. Remove from heat.

Discard parchment cartouche; divide salad and rice among 4 plates; top rice with fillets. Spoon sauce over fillets; garnish with thyme leaves. Serve immediately.