Preheat your oven to 350°F (175°C).
Bring a large pot of salted water to a boil. Add the cauliflower florets and cook for about 5 minutes, or until just tender. Drain and set aside in a colander to cool slightly.
In a medium saucepan, melt the butter over medium heat. Whisk in the flour and cook for about 1 minute, stirring constantly to create a roux.
Gradually whisk in the milk, continuing to stir until the mixture thickens, about 3-5 minutes.
Remove the saucepan from heat and stir in the shredded cheddar cheese, grated Parmesan cheese, garlic powder, onion powder, salt, and pepper until smooth.
In a mixing bowl, combine the breadcrumbs and olive oil, stirring until the breadcrumbs are evenly coated.
In the baking dish, combine the cauliflower florets with the cheese sauce, mixing well to ensure the florets are evenly coated.
Sprinkle the breadcrumb mixture over the top of the cheesy cauliflower bake.
Bake in the preheated oven for 25-30 minutes, or until the breadcrumbs are golden brown and the cheese is bubbly.
Remove from oven, let cool for a few minutes, and garnish with chopped parsley if desired. Serve warm.