Preheat oven to 375 degrees F (190 degrees C).
Butter a 13x9x2-inch casserole dish.
Slice crust from Country White Bread (1 pound) and cut into 1-inch cubes to yield 10 cups loosely packed cubes.
In a large bowl, toss bread with Olive Oil (1/4 cup), Fresh Thyme (4 teaspoon), Garlic (1 clove), and Salt and Pepper (1 pinch).
Spread evenly in a single layer on a large baking tray and bake for 18 to 20 minutes until golden, turning once or twice. Remove from oven and return bread cubes to same large bowl.
Heat Unsalted Butter (1/4 cup) in a large skillet until foamy.
Add Celery (1 1/2 cup), Onion (1 cup), Green Bell Pepper (3/4 cup), and Mushroom (1 pound) to skillet. Sauté until softened and most of the moisture has been cooked out, about 15 minutes.
Heat up the Frozen Spinach (5 ounce) to thaw and squeeze out excess moisture.
While vegetables are cooking, whisk together Egg (8), Heavy Cream (3 1/2 cup), spinach, Kosher Salt (2 teaspoon), and Ground Black Pepper (1 teaspoon).
Pour egg mixture over toasted bread cubes, add sauteed vegetables, Fresh Parsley (1/3 cup), and Gouda (1/3 cup) and stir to combine well. Pour into prepared dish, spreading out evenly.
Top with Parmesan Cheese (1/3 cup), then cover and chill for at least an hour to allow bread to absorb the custard.
Preheat oven to 350 degrees F (175 degrees C).
Bake uncovered for about 60 minutes until set and golden.
Remove from oven and allow to rest for 10-15 before serving.