Preheat the oven to 400°F. Season the cauliflower with 1 tbsp of oil and the salt in a large bowl.
Heat the remaining oil in a skillet for 3–4 minutes over medium-high heat. Add the cauliflower and let it brown, undisturbed, for 3–4 minutes. Turn and cook for 3 more minutes. Remove the cauliflower from the pan and set it aside. Let the pan cool slightly.
For the Mornay sauce, make a roux by melting the butter in the skillet over medium-low heat. Whisk in the flour and cook for 1 minute. Gradually add the milk, whisking constantly. Bring to a simmer and cook until thick, 2–4 minutes. Remove from the heat and add the white cheddar, ½ cup of the Gruyere, garlic powder, and nutmeg (if using), and mix until combined. Add the cauliflower to the sauce and mix well to coat. Top with the remaining Gruyere and the almonds. Bake, uncovered, until bubbly, about 14–16 minutes.
Remove from the oven and let stand for 4–5 minutes. Top with parsley.
