Preheat oven to 375°F. Coat a 1.5-qt baking dish with non-stick spray.
Combine breadcrumbs, melted butter, bacon crumbles, and various zests. Season with salt and pepper. Set aside.
Cook noodles in a pot of salted boiling water according to package directions; drain.
Sprinkle Old Bay on both sides of tuna. Sear tuna in oil in a sauté pan over medium-high heat until blackened, 2 minutes per side. Transfer tuna to a plate to cool; break into chunks.
Melt 2 tablespoons of butter in a skillet over medium-high heat; sauté leeks, mushrooms, garlic and thyme until leeks are tender, 5 minutes. Stir in flour; cook 1 minute. Add lemon, lime and orange juices; cook until evaporated. Reduce heat to medium.
Whisk in broth and cream; simmer 5 minutes. Stir in cheddar until it melts; season with salt and pepper. Toss noodles, tuna and broccoli with sauce; transfer to prepared dish.
Top casserole with breadcrumb mixture. Bake until top is golden-brown, around 15 minutes.