Preheat the oven to 400 degrees. Line 2 baking sheets with parchment paper or silicone baking mats.
Slice the eggplant into ½-inch slices. Brush both sides with olive oil and season with salt and pepper. Roast the eggplant for 30-35 minutes or until golden and soft, flipping the slices halfway through cooking. While the eggplant is roasting, make the sauce.
Combine the vinegar, honey, red pepper flakes, Fresno chilies, and ¼ teaspoon salt in a small saucepan. Bring to a boil then lower the heat and simmer until the mixture reduces and become syrupy, about 8 minutes.
Place the roasted eggplant on a serving platter and brush each side with the sauce. Spoon the rest of the sauce over top and sprinkle with green onions and walnuts.