In the cruet or small jar, shake together the lemon juice, dill, kosher salt, Dijon mustard, and black pepper. Allow the salt to dissolve, then add the extra-virgin olive oil and shake well. Set aside.
In the small pot, cover the lentils with water and bring to a boil. Reduce heat to low and continue cooking until the lentils are tender, but not mushy, about 30 minutes. Drain well, toss with the dressing, bell pepper, cucumber, and chopped red onions. Chill.
Rub the salmon with a little olive oil and black pepper on both sides. Grill over medium heat, turning half way through cooking, until the meat is firm and flaky, about 10 to 15 minutes, depending on the thickness of the filets. Remove from the grill and flake the meat.
On individual plates, arrange a layer of the spinach, then put a scoop or two of the lentil mixture on top. Arrange chunks of the salmon over the bed of lentils and sprinkle on fresh dill for garnish.