Pasta with Eggplant, Feta, and Mint

spaghetti or any other long thin Italian pasta (linguini or fettucini)

Salt

globe eggplant, peeled and cut into 1/2 to 3/4-inch cubes (about 2 1/2 cup of cubed eggplant)

olive oil, divided

medium red onion, cut in half lengthwise and thinly sliced crosswise (about 1 1/2 cups)

garlic, minced (1 Tbsp)

fresh baby spinach (3 ounces), roughly chopped

to 6 mint leaves (spearmint), minced

feta cheese, crumbled

Freshly ground black pepper