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 minced shallot. Dice potatoes into 1/2-inch pieces. Halve lemon. Place cranberries in a medium bowl with enough hot water to cover by 1 inch.

Heat a large drizzle of oil in a medium 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 another medium bowl. Season with salt and pepper. Rinse and wipe out pan.

Place potatoes in a medium pot with enough salted water to cover by 1 inch. Bring to a boil and cook until potatoes are tender, about 15 minutes. Drain and return to pot; mash with a potato masher or fork until smooth. Stir in sour cream, 1 TBSP butter, salt, and pepper. Keep covered until ready to serve.

Pat steak dry with a paper towel, then season all over with salt and pepper. Heat a drizzle of oil in same pan used to caramelize onion and shallot over medium-high heat. Add steak and cook to desired doneness, 4-8 minutes per side. Turn off heat. Remove steak from pan and set aside to rest for 5 minutes. Rinse and wipe out pan.

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

In a large bowl, toss together lettuce, cranberries (draining before adding), juice from half the lemon, and a drizzle of olive oil. Season with salt, pepper, and more lemon juice, if desired. Stir caramelized onion and shallot into potatoes. Divide salad, potatoes, and steak among plates. Top steak with sauce.