Wash and dry all produce. Halve, peel, and thinly slice onion. Halve, peel, and thinly slice shallot; mince a few slices until you have 3 TBSP. Dice potatoes into 1/2-inch pieces. Halve lemon. Place cranberries in a medium bowl with enough hot water to cover by 1 inch; set aside.

Heat a large drizzle of oil in a large pan over medium-high heat. Add onion and sliced shallot and cook, stirring, until browned and softened, 15-20 minutes. (TIP: If pan seems dry, add a splash of water.) Turn off heat; transfer to a second medium bowl. Season with salt and pepper. Wash out pan.

Place potatoes in a large pot with enough salted water to cover by 2 inches. Boil until potatoes are tender, 15-20 minutes, then drain and return to pot. Mash until smooth; stir in sour cream, 1 TBSP butter (2 TBSP for 4 servings), salt, and pepper. Keep covered until ready to serve.

Pat steak dry with paper towels, then season all over with salt and pepper. Heat a drizzle of oil in pan used to caramelize onion and shallot over medium-high heat. Add steak and cook to desired doneness, 4-7 minutes per side. Turn off heat. Transfer to a plate to rest for 5 minutes. Wash out pan.

Melt 1 TBSP butter in same pan over low heat. Add minced shallot and cook, stirring, until softened, 1-2 minutes. Stir in demi-glace and 1/4 cup water (1/3 cup for 4 servings). Simmer until slightly thickened, 1-2 minutes. Turn off heat. Stir in another 1 TBSP butter and any resting juices from steak. Season with salt and pepper.

In a large bowl, toss together mixed greens, cranberries (draining before adding), juice from 1 lemon half, and a drizzle of olive oil. Season with salt, pepper, and more lemon juice to taste. Stir onion and shallot into potatoes. Divide salad, potatoes, and steak between plates. Top steak with sauce.