Begin by preparing the creamy sauce. Melt the butter in a saucepan over medium heat. Add in the chopped onions and cook until they are translucent. Slowly pour in the heavy cream while stirring constantly to avoid lumps. Allow the mixture to thicken for approximately 5 minutes, then add 2 tablespoons of cornstarch to further thicken the sauce. Stir in 1 teaspoon of chicken bouillon and let it simmer for an additional 2 minutes. Once thickened, turn off the heat and set the sauce aside.

Next, cook the seafood filling. In a separate pan, melt 4 teaspoons of butter over medium heat. Add in the shrimp and cook until they turn pink. Then, stir in the lump crab meat and cook for an additional 2-3 minutes. Add in some lemon juice and stir well.

Now it's time to assemble the chimichangas. Begin by preheating your oven to 375°F (190°C). Lay a tortilla flat on a clean surface and add a generous amount of the seafood filling in the center. Top with sliced avocados and shredded lettuce. Wrap the tortilla tightly, tucking in the ends to create a burrito-like shape. Place the chimichangas on a baking sheet lined with parchment paper, seam side down. Brush each chimichanga with melted butter for a golden and crispy finish.

Bake the chimichangas in the preheated oven for 20 minutes, or until the tortilla is crispy. While they are baking, prepare your toppings by mixing together diced onions, chopped tomatoes, cilantro, and jalapenos in a bowl.

Once the chimichangas are done, remove them from the oven and allow them to cool for a few minutes. Serve with the creamy sauce on top and sprinkle with your favorite toppings.