Melt the butter in a saucepan over low heat; add onion and sauté until translucent. Mix together the flour, salt, and pepper in small bowl. Whisk the flour mixture into the saucepan. Cook the mixture; stirring frequently until a light brown roux forms. Slowly whisk in the milk, increasing the heat to medium. When the sauce slightly thickens, add the lobster meat. Continue cooking until it is bubbly. Remove from the heat and whisk in sour cream. Serve immediately.