Start by draining the thawed chopped spinach well, pressing it between paper towels to remove any excess moisture.

Set it aside.

In a large skillet over medium heat, melt the butter and add the olive oil.

Sauté the sliced onions for about 5 minutes until they start to become tender.

Stir in the brown sugar and continue to cook, occasionally stirring, for an additional 20 minutes or until the onions caramelize.

Reserve ¼ cup of these caramelized onions for later use.

In a separate bowl, whisk together the eggs and the next 4 ingredients (half-and-half, salt, nutmeg, and pepper) until the mixture is well blended.

Stir in the drained spinach.

Evenly spoon the caramelized onions (excluding the reserved portion) into 6 lightly greased 6-ounce custard cups.

Sprinkle the crumbled Gorgonzola cheese evenly over the onions.

Pour the egg and spinach mixture evenly over the cheese layer in the custard cups.

Place the custard cups in a 13x9-inch baking pan and add hot water to the pan until it reaches a depth of 1 inch.

Bake the custards at 350°F (175°C) for 30 to 40 minutes or until they are almost set.

Remove the custard cups from the pan and let them stand for 10 minutes.

Afterward, unmold them and top each custard with a portion of the reserved caramelized onions.

If desired, garnish the Spinach and Gorgonzola Cheese Custards with additional crumbled Gorgonzola cheese.