Wash all vegetables thoroughly and dry them before cutting. Dice squash, peppers, and onions into similar sizes so they cook evenly. Steam carrots briefly so they soften enough to cook at the same pace as other vegetables.
Place a large skillet over medium heat and add olive oil. Allow the oil to heat until it lightly shimmers, signaling the pan is ready. Proper heat prevents vegetables from steaming and helps them develop flavor.
Add all prepared vegetables into the hot skillet and stir to coat them in oil. Season with salt and pepper, then cook for 3–5 minutes while stirring occasionally. The vegetables should become tender but still slightly crisp when pierced with a fork.