Bring the lentils, water, and salt to a boil in a medium saucepan over high heat. Reduce the heat to low and simmer until the lentils are tender but still have a slight bite, about 20 to 25 minutes.

Drain the lentils well in a fine-mesh strainer and set aside to cool slightly.

Place the baby spinach and water in a microwave-safe bowl. Cover tightly and microwave until the spinach is wilted and its volume is halved, about 3 to 4 minutes.

Leave the bowl covered for 1 minute to finish steaming. Transfer the spinach to a colander and gently press with a spoon to release the excess liquid.

Transfer the spinach to a cutting board and roughly chop. Return the chopped greens to the colander and press one more time—removing this extra moisture prevents the salad from becoming watery.

Whisk the vinegar, olive oil, and shallot together in a large bowl. Add the warm lentils and toss until the dressing is absorbed.

Fold in the chopped spinach and parmesan. Season with salt and pepper to taste.

Scatter the toasted walnuts over the top and serve immediately while the cheese is fresh.