Remove pulp from eggplants; cut pulp from one eggplant into 1/4-inch cubes and discard the rest. Coat eggplant shells with cooking spray; sprinkle with 1/4 teaspoon salt. On a baking sheet lined with parchment paper, place eggplant shells open side down. In a bowl, coat eggplant cubes with cooking spray and toss with 1/4 teaspoon salt; place in one layer on a second baking sheet lined with parchment paper.
Bake eggplant shells and cubes until tender, 10 minutes.
Remove eggplant; reduce oven to 350°. In a medium nonstick skillet over medium heat, heat oil. Cook onion, celery and carrot until soft,5 minutes.
Add garlic; cook, 1 minute.
Add lentils, eggplant cubes, broth, vinegar, pepper and remaining 1/4 teaspoon salt; reduce heat to simmer, stirring occasionally, 5 minutes. Stir in chopped mint. Flip shells on baking sheet and stuff with lentil-eggplant mixture; cover with aluminum foil and bake until warm; 10 minutes. Top with cheese and mint leaves.