Drain spinach well, pressing between paper towels to remove excess moisture; set aside.
Melt butter in a large skillet over medium heat; add oil and onion, and sauté 5 minutes. Stir in sugar, and cook, stirring occasionally, 20 more minutes or until onion is caramel colored. Reserve 1/4 cup onion; spoon remaining onion mixture evenly into 6 lightly greased 6-ounce custard cups; sprinkle with cheese.
Whisk together eggs and next 4 ingredients until blended. Stir in spinach. Spoon evenly over cheese, and place custard cups in a 13- x 9-inch pan. Add hot water to pan to a depth of 1 inch.
Bake at 350° for 30 to 40 minutes or until almost set. Remove cups from pan. Let stand 10 minutes; unmold and top with reserved onion. Garnish, if desired.
*1 (4-ounce) package crumbled blue cheese may be substituted for 5 ounces Gorgonzola.
Note: Savory Spinach-Gorgonzola may be baked in a lightly greased 9-inch square baking dish.