Melt butter in a large skillet over medium heat; add diced onion, and cook, stirring occasionally, 30 minutes or until caramel colored. Remove mixture from heat.
Process onion, half-and-half, and remaining ingredients in a food processor or blender until smooth, stopping to scrape down sides.
Pour into 6 lightly greased 6-ounce custard cups. Cover cups with aluminum foil, and place in a 13- x 9-inch pan. Add hot water to pan to depth of 1/2 inch.
Bake at 350° for 45 minutes or until a knife inserted in center comes out clean. Remove from hot-water bath, and cool slightly; invert onto individual plates.