Adjust rack to middle position and preheat oven to 400 degrees. Bring a medium pot (large pot for 4 servings) of salted water to a boil. Wash and dry produce.

Trim, peel, and halve carrot lengthwise; cut crosswise into ¼-inch pieces. (TIP: If your carrot is on the larger side, quarter lengthwise before cutting crosswise.) Halve, peel, and dice onion into ¼-inch pieces. Slice celery into ¼-inch pieces. Pick parsley leaves from stems. Finely chop stems; roughly chop leaves. Quarter lemon.

Drain potatoes; pat dry with paper towels.

In a small bowl, combine panko, cheddar, ½ tsp Old Bay Seasoning (1 tsp for 4), and a large drizzle of olive oil.

Heat a drizzle of olive oil in a large pan over medium-high heat. Add carrot, onion, celery, and parsley stems. Cook, stirring occasionally, until beginning to soften, 4-5 minutes.

Add remaining Old Bay Seasoning; cook, stirring, until fragrant, 30 seconds. Stir in vinegar; cook until mostly evaporated, 30-60 seconds. Add cream sauce base; cook, stirring, until sauce has thickened, 10-20 seconds. Add cheese roux concentrate; cook, stirring, until sauce has thickened, 10-20 seconds more.

Turn off heat; set aside.

Open and drain scallops*; remove and discard any small, tough pieces attached to scallops. Rinse scallops and shrimp* under cold water and pat dry with paper towels.

Once water is boiling, carefully add scallops and shrimp to pot. Cook until cooked through, 1 minute, then immediately drain and rinse under cold water to stop the cooking process. Shake off any excess water.

Add scallops, shrimp, and half the potatoes to pan with sauce. Gently stir to evenly coat. TIP: Depending on the size of your pan, you may need to transfer the sauce, seafood, and potatoes to a large bowl and toss to combine.

Lightly coat the insides of two small oven-safe bowls or ramekins (four for 4 servings) with butter.

Divide seafood mixture between prepared bowls. Arrange remaining potatoes on top. Top with panko mixture in an even layer.

Transfer gratins to a baking sheet. Bake on middle rack until panko mixture is lightly browned and sauce is slightly bubbly, 15-18 minutes.

Carefully remove from oven; let rest for 5-10 minutes before serving.

Carefully place gratins on plates. Garnish with remaining parsley leaves. Serve salad alongside gratins and any remaining lemon wedges on the side.