Wash and dry all produce. Adjust rack to upper position and preheat oven to 425 degrees. Halve Yukon gold and sweet potatoes lengthwise, then cut into 1/3-inch-thick wedges (like steak fries). Toss in a large bowl with 1 TBSP oil. Season with salt and pepper. Spread out on a baking sheet. Roast in oven until lightly browned, 25-30 minutes.

Halve and peel shallot, then mince until you have 2 TBSP. Pick parsley leaves from stems; discard stems. Finely chop leaves. Halve tomatoes. Halve or quarter any large mini bell peppers through stem ends. Pat steak dry with a paper towel and season all over with salt and pepper.

At about the same time that potatoes have roasted 15 minutes, heat a large drizzle of oil in a large pan over medium-high heat. Add steak and cook to desired doneness, 4-6 minutes per side. Transfer to a plate, cover loosely with foil, and let rest. Pour out any excess oil from pan and wipe clean.

Heat another drizzle of oil in pan used for steak over medium-high heat. Add mini bell peppers. Cook, tossing, until slightly softened and starting to char, 4-5 minutes. Toss in tomatoes and cook until softened and their juices begin to release, 1-2 minutes more. (TIP: If pan is dry, add 1-2 TBSP water.) Season with salt and pepper. Remove from pan and set aside in a medium bowl.

Rinse out pan and wipe dry with a paper towel. Heat a drizzle of oil in same pan over medium heat. Add minced shallot. Cook, tossing, until softened and translucent, 1-2 minutes. Stir in 1/4 cup water and demi-glace, then bring to a boil. Let reduce slightly, about 2 minutes. Remove pan from heat. Add half the parsley and 1 TBSP plain butter (save the garlic herb butter for the next step). Stir to melt.

Slice steak against the grain. Stir any juices released by steak into sauce in pan. Divide potatoes, veggies, and steak between plates. Spoon sauce over steak and dollop with garlic herb butter, if desired. Sprinkle with remaining parsley and serve.