Method. Preheat the oven to 425 degrees Fahrenheit. To prepare the veggies: In a large skillet over medium heat, warm the olive oil over medium heat. Once shimmering, add the carrots, bell pepper, zucchini, yellow onion, and salt. Cook, stirring every couple of minutes, until the veggies are golden on the edges, about 8 to 12 minutes. Add a few large handfuls of spinach. Cook, stirring frequently, until the spinach has wilted. Repeat with remaining spinach and cook until all of the spinach has wilted, about 3 minutes. Remove the skillet from the heat and set aside. Meanwhile, to prepare the tomato sauce: Pour the tomatoes into a mesh sieve or fine colander and drain off the excess juice for a minute. Then, transfer the drained tomatoes to the bowl of a food processor. Add the basil, olive oil, garlic, salt, and red pepper flakes.