Heat 3 Tbsp. extra-virgin olive oil in a medium Dutch oven or very large skillet over medium-high. Add ¼ tsp. crushed red pepper flakes and cook, stirring, until fragrant, about 30 seconds. Add zucchini mixture (unwrap the balled-up towel over the pot for easiest transfer) and 2 tsp. Diamond Crystal or 1¼ tsp. Morton kosher salt and stir to coat. Season generously with freshly ground black pepper. Cook, stirring occasionally, until zucchini mixture is jammy, thick, and starting to stick to bottom of pot, 15–20 minutes.