- 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.