Make chimichurri: In a mini food processor, pulse parsley, oregano, and garlic until finely chopped. Scrape herb mixture into a small bowl and stir in olive oil, vinegar, red pepper flakes, and season with salt. Let sit so flavors can meld while cooking eggplant steaks and frites.
Generously season both sides of eggplant slices with salt (about 1½ teaspoons). Arrange eggplant on a wire rack set over a rimmed baking sheet and let drain for 30 minutes. Rinse eggplant and pat dry. Brush both sides of eggplant with 2 tablespoons oil and season with salt and pepper.
Meanwhile, place potatoes in a large pot and add enough cold water to cover by 1 inch. Season the water with 1 teaspoon salt and bring to a boil. Reduce heat to medium and simmer until potatoes are just tender, about 5 minutes. Drain and transfer potatoes to a large bowl; pat dry and toss with 2 tablespoons oil.
Grill vegetables: Preheat grill or grill pan to medium-high. Grill eggplant and potatoes, turning halfway through, until golden and tender, about 5 minutes for the potatoes and 8 minutes for the eggplant, transferring vegetables to a platter or plate as they are done. Season potatoes with salt.
Serve eggplant steaks and frites with a generous dollop of chimichurri.