Cook the lentils: Set a medium saucepan on the stove and add the lentils with 4 cups of water.

Cook the lentils: Bring the water to a boil over high heat.

Cook the lentils: Lower the heat to a gentle simmer and cook the lentils for 22 to 25 minutes, until they are tender but still hold their shape.

Start the dressing: While the lentils cook, put the sliced red onion into a large serving bowl.

Start the dressing: Squeeze the juice from one lemon half into the bowl with the onion.

Start the dressing: Add the Dijon mustard, olive oil, and 1/2 teaspoon of the salt to the bowl. Whisk until the dressing looks smooth.

Start the dressing: Let the onion sit in the dressing while the lentils finish cooking. This takes away some of the sharp bite.

Prepare the toppings: Put the drained tuna into a small bowl or onto a plate and break it into large flakes with a fork.

Prepare the toppings: Chop the parsley if you have not already, and set aside 1 tablespoon for the final finish.

Build the bowls: Drain the cooked lentils in a colander and let the water run off well for a few seconds.

Build the bowls: Transfer the hot drained lentils to the large bowl with the dressing and onion. Stir well so the lentils are coated.

Build the bowls: Add the baby spinach to the bowl and toss until the spinach softens and shrinks slightly from the heat of the lentils, about 1 minute.