Adjust rack to top position and preheat oven to 425 degrees. Wash and dry all produce. Slice potatoes into 1/2-inch-thick wedges. Halve, peel, and thinly slice onion. Halve lemon. Thinly slice mozzarella. In a small bowl, combine sour cream and 1/4 tsp truffle zest (1/2 tsp for 4 servings; we’ll use the rest later).

Toss potatoes on a baking sheet with a drizzle of olive oil; season with salt and pepper. Roast on top rack until browned and tender, 20-25 minutes.

Meanwhile, heat a drizzle of olive oil in a large pan over medium-high heat. Add onion; cook, stirring occasionally, until browned and softened, 8-10 minutes. Add 2 TBSP water and 1 tsp sugar (2 tsp for 4); cook, stirring, until caramelized and jammy, 2-3 minutes. Season with salt and pepper. Turn off heat. Transfer to a small bowl and stir in remaining truffle zest. Wipe out pan.

Place bread on your work surface. Top half the slices with an even layer of mozzarella and onion; top with remaining slices of bread. Melt 1 TBSP butter in pan used for onion over medium heat. Add sandwiches; cook until golden brown on first side, 3-5 minutes. Flip and add another 1 TBSP butter to pan; cook until bread is golden brown and cheese is melted, 3-5 minutes more. TIP: For 4 servings, you may need to cook in batches, adding 1 TBSP butter for each side.

Meanwhile, toss mixed greens in a large bowl with a large drizzle of olive oil and a squeeze of lemon juice to taste. Season with salt and pepper.

Slice grilled cheese sandwiches in half on a diagonal. Divide between plates with potato wedges and salad on the side. Serve with truffled sour cream for dipping.