Melt butter in a large skillet over low heat.
Add chopped onion, minced garlic, and fresh ginger to the pan. Cook for 10 minutes or until tender and fragrant.
In a small bowl, combine flour, curry powder, and salt.
Add the flour mixture to the skillet and cook, stirring constantly, for 5 minutes to create a roux.
Increase heat to medium and gradually whisk in warm milk, stirring continuously until smooth and lump-free.
Reduce heat to low, cover, and simmer for 20 minutes to allow flavors to develop.
Strain the mixture through a fine-mesh sieve into a clean saucepan to guarantee smoothness.
Add whipping cream, cooked shrimp, and crabmeat to the strained sauce. Gently combine over low heat.
Add fresh lemon juice to taste and heat through without boiling.
Serve immediately over cooked rice and garnish with lemon wedges.