Wash and dry all produce. Peel potatoes, then cut into 1/2-inch cubes. Halve, peel, and dice onion. Trim, then thinly slice scallions, keeping greens and whites separate. Mince garlic.

Heat a large drizzle of oil in a large pot over medium-high heat. Add garlic, onion, and scallion whites. Cook, tossing, until very soft, about 5 minutes. Season with salt and pepper. Stir in flour and cook until pasty, about 1 minute. Slowly stir in milk, adding just a little at a time, until incorporated.

Once all of the milk is incorporated, add potatoes, 21/4 cups water, and stock concentrates to pot. Bring to a boil, then lower heat and reduce to a simmer. Cook, stirring occasionally, until potatoes are easily pierced by a knife, about 15 minutes.

While soup simmers, heat a large pan over medium-high heat. Add pancetta and cook, tossing often, until crispy, 5-7 minutes. Transfer to a paper-towel-lined plate. Meanwhile, core and halve apple, then slice into thin half-moons. Halve lemon, then squeeze 11/2 TBSP juice into a large bowl. Add mustard and 2 TBSP olive oil to bowl and whisk to combine. Season with salt and pepper. Set aside.

Once potatoes are tender, lightly mash mixture in pot until mostly smooth but a few potato chunks still remain. Remove pot from heat and let cool slightly, then stir in half the sour cream and half the cheese. Season generously with salt and pepper.

Add apple and lettuce to bowl with dressing and toss to coat. Season with salt and pepper. Divide soup between bowls and top with pancetta, remaining cheddar, and scallion greens. Dollop with remaining sour cream. Serve with salad on the side.