How to Make It
In a large saucepan, cover the lentils with at least 2 inches of water and bring to a boil. Simmer over moderate heat until just tender, about 20 minutes. Remove from the heat, add a generous pinch of salt and let stand for 5 minutes. Drain well and spread the lentils on a baking sheet to cool.
Rinse out the saucepan and put a steamer basket in it. Add 1 inch of water and bring to a boil. Scatter the beets in the basket, cover and steam until tender, about 10 minutes. Let the beets cool.
In a large bowl, whisk the vinegar with the shallot, mustard and 1/4 cup of the olive oil. Season with salt and pepper. Add the lentils, beets, frisée and endives and toss to coat. Season with salt and pepper and toss again.
Season the salmon with salt and pepper. In a large nonstick skillet, heat the remaining 1 tablespoon of olive oil until shimmering. Add the salmon, skin side down, and press gently with a spatula to flatten. Cook over moderately high heat until the skin is browned and crisp, about 3 minutes. Flip the salmon and cook until it is medium within, about 3 minutes longer. Transfer the salmon to plates and serve with the lentil salad.