Place a medium-sized sauté pan over medium high heat, add 2 tbsp butter, sauté onion and garlic for 4 minutes and quickly add fresh spinach and thyme. Cook until spinach is green and soft. Finish with salt and pepper to taste. Place in the refrigerator so that the spinach is cool for later.
Preheat oven to 375°F. Beat the egg whites with a mixer for 5 to 7 minutes, until they form soft peaks, resembling a meringue. Meanwhile, in a medium-sized sauce pan, warm up milk and cream on medium heat. When milk is warm, add ¼ cup of corn meal, nutmeg, salt and pepper. With a whisk, beat the mixture until the corn meal thickens, around 4 to 6 minutes; it will have the consistency of thick pancake batter. Transfer into a large bowl.
When the corn meal mixture is slightly cool, whisk the 3 egg yolks into the batter and add the parmesan cheese. With a spatula, add 1/3 of the egg whites to the corn meal batter, folding it in from top to bottom gently so that the batter keeps as much air and volume as possible. Repeat with the leftover egg whites. Add the shredded Old Cheddar Cheese as gently as possible to keep as much air in the batter.
Finish by adding the spinach mixture to the soufflé batter.
Butter the inside of a 9” pie pan with the remaining butter and sprinkle the leftover corn meal at the bottom, just like you would do when baking a cake. Pour in the soufflé batter and bake in the oven for 25 to 30 minutes without opening the door. Remember to be patient, it will pay off. The soufflé should be golden brown, nice and fluffy. Serve immediately.