• In a fine-mesh strainer, rinse the lentils.

In a small sauce pot, combine the lentils, bay leaf, and 4 cups lightly salted water. Bring to a boil, reduce to a simmer, and cook, stirring occasionally, until the lentils are tender, 18 to 20 minutes. Drain the lentils and rinse under cold water. Discard the bay leaf.
While the lentils simmer, prepare the onion and eggplant.

• Peel and coarsely chop the yellow onion.
• Remove the stem from the eggplant and cut in half lengthwise. Cut each half into ½-inch-wide strips, then crosswise into ½-inch cubes.
• In a medium bowl, toss the eggplant with the paprika and season with salt and pepper. Let stand while you cook the onion.

In a large frying pan over medium-high heat, warm 1 teaspoon oil until hot but not smoking. Add the onion, season with salt and pepper, and cook, stirring occasionally, until lightly browned and starting to soften, 2 to 3 minutes. Transfer to a plate.
In the same pan, if dry, add 1 teaspoon oil and warm over medium-high heat until hot but not smoking. Add the eggplant (do not clean the bowl). Cook, stirring occasionally, until starting to brown, 3 to 5 minutes. Transfer to a separate plate.
While the onion and eggplant cook, prepare the plum relish and the garnishes.

• Cut the plums in half and remove the pits; coarsely chop the plums.
• Strip the parsley leaves from the stems; coarsely chop the leaves.

In a small bowl, stir together the plums, parsley, and 2 teaspoons oil; season to taste with salt and pepper.

• Coarsely chop the hazelnuts.
• Trim the ends from the celery. Thinly slice the celery on the diagonal.
• Cut the lemon into wedges.

In the bowl used for the eggplant, stir together the lentils, onion, and arugula; season to taste with salt and pepper.

Transfer the lentils to individual bowls. Arrange the eggplant, plum relish, hazelnuts, and celery in separate piles on top. Serve the lemon wedges on the side.