Bring a large pot of well-salted water to a boil. Cook the elbow macaroni about 1 minute less than package directions for al dente, then drain.

While the pasta cooks, gently toss the lump crab meat with 1 teaspoon Old Bay and the lemon juice in a bowl. Set aside.

In a heavy-bottomed saucepan, melt the butter over medium heat. Whisk in the flour and cook for about 90 seconds until pale golden and fragrant.

Slowly whisk in the warmed milk and cream. Continue whisking over medium heat until the sauce thickens enough to coat the back of a spoon, about 5 minutes.

Reduce heat to low. Whisk in the cream cheese until melted, then add the cheddar and Gruyère in handfuls, whisking until smooth. Stir in the Dijon, garlic powder, remaining ½ teaspoon Old Bay, salt, and pepper.

Remove the sauce from heat. Stir in the drained pasta, then gently fold in the seasoned crab meat using a spatula. Pour into a buttered 9x13-inch baking dish.

Mix the panko, melted butter, and parmesan, then sprinkle evenly over the top. Bake at 375°F for 18–20 minutes, until golden and bubbling at the edges.

Let rest for 5 minutes before serving. Garnish with chopped chives.