Preheat oven to 400°F (200°C). Line a baking sheet with parchment paper.
Place broccoli, zucchini, and bell pepper on the sheet. Drizzle with olive oil, season with salt and pepper, and toss to coat. Roast 20–25 minutes until tender and slightly charred.
Meanwhile, season pork tenderloin with salt, pepper, and Italian seasoning.
Heat olive oil in a large oven-safe skillet over medium-high heat. Sear pork on all sides until golden, about 2–3 minutes per side.
Reduce heat to medium. Add butter and garlic to the skillet. Spoon garlic butter over the pork as it cooks.
Transfer skillet to oven (place alongside veggies if space allows) and roast pork 12–15 minutes, or until internal temperature reaches 63°C (145°F).
Remove pork, rest for 5 minutes, then slice into medallions.
Serve pork slices with roasted veggies and drizzle with garlic butter sauce. Garnish with parsley.