Adjust rack to top position and preheat oven to 425 degrees. Wash and dry produce.

Dice potatoes into ½-inch pieces. Toss on a baking sheet with a large drizzle of oil, salt, and pepper. Spread out in a single layer.

Roast on top rack until golden brown, 20-25 minutes, tossing halfway through.

While potatoes roast, pat steak* dry with paper towels; season with salt and pepper.

Heat a drizzle of oil in a large pan over medium-high heat. Add steak and cook to desired doneness, 4-7 minutes per side. TIP: If steak has a layer of fat on one side, use tongs to flip and sear until fat is rendered and crispy.

Turn off heat; transfer steak to a cutting board to rest. Wipe out pan.

Heat a drizzle of oil in pan used for steak over medium heat. Add shallot; cook, stirring, until softened, 1-2 minutes.

Stir in cream sauce base, mustard, stock concentrate, ¼ cup water, and ½ tsp herbes de Provence (⅓ cup water and 1 tsp herbes de Provence for 4 servings). Simmer until slightly thickened, 1-2 minutes more. (Save remaining herbes de Provence for another use.)

Turn off heat. Stir in parsley and 1 tsp vinegar (2 tsp for 4). 

In a large bowl, whisk together remaining vinegar, 1 tsp sugar (2 tsp for 4 servings), a large drizzle of olive oil, salt, and pepper.

Add mixed greens and toss until coated.

Divide steak, potatoes, and salad between plates. Spoon sauce over steak and garnish salad with walnuts. Serve.