Step 1

Peel and halve 1 medium shallot lengthwise. Lightly smash 2 garlic cloves. Cut 1 medium lemon into 1/4-inch thick slices and remove any seeds.

Step 2

Season both sides of a 1-pound skinless salmon fillet with 1 teaspoon kosher salt. Place the salmon in a large straight-sided skillet or medium pot and cover with 2 cups full-fat buttermilk and 1 cup low-sodium vegetable stock. (It’s OK if the salmon is not fully submerged.) Add the shallot, garlic, lemon slices, 5 sprigs fresh thyme, and 2 bay leaves.

Step 3

Bring to a simmer over medium heat, then reduce the heat to low so the poaching liquid is barely simmering. You'll see some white scummy foam collecting on the surface and the liquid will curdle. Cover and cook until the salmon is pale pink, opaque, and firm when pressed, 11 to 12 minutes. Remove the pan from the heat and let sit covered for 5 minutes.

Step 4

Use a slotted spoon to transfer the salmon onto a serving plate. Remove the shallot, garlic, lemon slices, bay leaves, and thyme, and reserve for garnishing if desired.

Step 5

Add 1 teaspoon Dijon mustard to the poaching liquid and place the pot back onto the heat. Bring to a simmer over medium heat and cook until the poaching liquid is milky and reduced by half, 9 to 11 minutes. Strain through a fine-mesh strainer into a large bowl. Serve the sauce spooned over the salmon. Garnish with the reserved aromatics if desired.