Step 1

Rinse and pick over the lentils in a strainer under running water. Put them in a large pot with the shallot, 4 cups water, and a generous pinch of salt. (You’re going to need the space later so resist the urge to use a saucepan.) Bring to a boil, then reduce the heat so the liquid bubbles steadily. Cover and cook, stirring once or twice, for 15 minutes before checking for doneness. You want them to be tender without getting mushy and breaking apart. Keep checking every couple minutes until they’re ready.

Step 2

Take the salmon out of the refrigerator and divide the fillet (or fillets) crosswise into portions if you’d like. (Sharp kitchen scissors make this tidy work.) Remove pin bones now with pointed pliers if you’d like or alert everyone at the table to do it as they eat. Grab a large non-stick skillet and have it handy.

Step 3

Drain lentils and cool quickly under cold tap water. (No need to rinse out the pot.) Fish out the shallot, chop it, and add it to the pot with the olive oil and turn the heat to medium. When the shallot starts sizzling (it won’t color much), adjust the heat to medium-low and squeeze in 2 tablespoons lemon juice. Add the coarse mustard, black pepper, and tarragon. Taste for salt and add more lemon juice if you’d like. Stir to combine; the dressing won’t emulsify but that’s okay. Spoon 4 tablespoons into a small bowl and reserve.

Step 4

Return the lentils to the pot and toss to coat with the dressing in the pot. Add the spinach leaves and stir until they just start to darken and soften. Cover, remove from heat, and let the salad sit while you cook the salmon.

Step 5

Put the dry skillet over high heat until it’s scorching hot. Sprinkle the bottom  with salt, then add the salmon, skin side down. Cook, lowering the heat a tad if you smell burning, until the skin is well browned and the fillets release easily from the pan, about 5 minutes. Turn the salmon and cook 1 additional minute (more if you like your salmon well done). Taste the lentils and adjust the seasoning, adding more olive oil, lemon, or mustard as needed. Drizzle the salmon with the reserved dressing and serve.