Step 1

Make the lentils: In a medium pot over high heat, combine the water and lentils and bring to a boil. Reduce the heat so the water is at a simmer, then cook, uncovered, until the lentils are firm-tender, about 20 minutes. Drain; you should have about 3 cups of cooked lentils.

Step 2

In a large skillet over medium heat, heat the oil until shimmering. Add the shallots and cook until softened, about 1 minute. Stir in the garlic and cook just until aromatic, 30 seconds more. Stir in the thyme, then add the lentils, lemon zest and juice, mustard, salt and pepper and stir to combine. Add the tomatoes and spinach, reduce the heat to medium-low, cover and cook, stirring occasionally, until warmed through and the spinach wilts, 2 to 3 minutes. Remove from the heat.

Step 3

Make the salmon: In a small bowl, stir together the mustard, lemon juice, honey and pepper. Preheat a nonstick or well-oiled grill or grill pan over medium-high heat. Place the salmon, skin side up, on the grill and cook for 3 minutes. Gently turn the fillets over, skin side down, brush with the mustard glaze and cook for 3 minutes more for medium-rare, or for 5 to 6 minutes if you prefer the fish cooked through. (If you don’t have a grill or a grill pan, follow the same cooking instructions using a cast-iron skillet, but add a slick of oil to the pan so the fish can sear in it.)

Step 4

Divide the lentil mixture among 4 plates, top each with a salmon fillet and serve.