Toss the eggplant with 3 tablespoons oil and spread it out on a baking sheet. Roast until tender and starting to brown, about 20 to 25 minutes in a 425F oven, tossing once halfway through.
While the eggplant roasts, heat the remaining 3 tablespoons oil in a large, deep skillet over medium to medium-high heat. Add the onion and celery and cook until softened and starting to brown in spots, about 8 minutes, stirring occasionally. Add the garlic and tomato and sauté until the garlic is fragrant, about 2 minutes, stirring constantly. Stir in the stock, turn the heat down to medium-low to low, cover the skillet, and cook until the tomatoes are starting to soften, about 8 minutes, stirring occasionally.
Add the eggplant to the tomato mixture and cook (covered) over low until the eggplant is very soft and most of the liquid is absorbed, about 10 minutes, stirring occasionally.
Add the vinegar, honey, salt, black pepper, and crushed red pepper flakes and cook (uncovered) until thickened, about 2 to 5 minutes.
Serve warm, room temperature, or cold topped with pine nuts and basil.