Peel and slice the carrots into 1/4-inch coins or matchsticks so they cook evenly (about 2 cups). Trim broccoli into bite-sized florets (about 2 cups). Core and seed the red bell pepper, then cut into 1/2-inch strips (about 1 cup). Slice the zucchini into 1/2-inch half-moons (about 1 cup). Thinly slice half a yellow onion to measure about 1/2 cup. Mince 2 garlic cloves.

Heat pan

Place a large skillet or sauté pan over medium-high heat. Add 2 tablespoons olive oil and 1 tablespoon butter (if using) to the hot pan and let the butter melt and foam slightly but not brown.

Sauté aromatics and onions

Add the sliced onion to the pan and sauté for 2–3 minutes, stirring occasionally, until it softens and becomes translucent. Add the minced garlic and cook for 30–45 seconds until fragrant, taking care not to burn it.

Add firm vegetables

Add the carrots and broccoli to the pan first, spreading them into a single layer as much as possible. Cook, stirring occasionally, for 3–4 minutes to get a light sear on the broccoli and begin to soften the carrots.

Add remaining vegetables and deglaze

Add the red bell pepper and zucchini to the pan. Pour in 1/4 cup vegetable broth or water to deglaze the pan and create a little steam to finish cooking the vegetables. Cover the pan with a lid and reduce heat to medium. Steam for 3–4 minutes until vegetables are tender-crisp (test with a fork).

Season and finish

Remove the lid and increase heat to medium-high for 30–60 seconds to evaporate excess liquid. Season with 1 teaspoon salt, 1/2 teaspoon freshly ground black pepper, and 1/4 teaspoon red pepper flakes if using. Stir in 1 tablespoon lemon juice and 2 tablespoons chopped fresh parsley. Taste and adjust seasoning as needed.

Serve

Transfer the vegetable mix to a serving dish. Optionally dot with an additional 1/2 tablespoon butter for gloss and richness. Serve warm as a side dish.