Instructions:
- If using fresh spinach, wilt it down. Crucially, squeeze ALL excess moisture out of the spinach using a clean tea towel. Roughly chop the squeezed spinach and set aside. Sauté the diced onion in 3 Tbsp butter until translucent (about 5 minutes). Add garlic and cook for 1 minute until fragrant. Remove from heat and set aside with the spinach.
- Wipe out the saucepan. Melt the remaining 4 Tbsp of butter over medium heat. Whisk in the flour constantly for 1-2 minutes to form a smooth roux. Do not allow it to brown significantly.
- Gradually whisk in the warmed milk, a splash at a time, ensuring no lumps form. Continue whisking until the sauce thickens enough to coat the back of a spoon (about 5-7 minutes). Bring to a gentle simmer, then reduce heat to low.
- Stir in the nutmeg, Dijon mustard, salt, and pepper. Remove the sauce from the heat entirely. Whisk in the grated Gruyère and Parmesan until smooth.
- Let the cheese sauce cool for 2 minutes, then vigorously whisk in the two lightly beaten eggs until fully incorporated. Gently fold in the prepared spinach and the sautéed onion/garlic mixture. Taste and adjust seasoning.
- Preheat your oven to 375°F (190°C). Lightly grease a 9x13 inch casserole dish and pour the spinach mixture evenly into it.
- In a small bowl, toss the Panko breadcrumbs with the 1 Tbsp of melted butter. Sprinkle this mixture evenly over the top of the casserole.
- Bake for 35–40 minutes, or until the topping is golden brown, the edges are bubbling, and the centre is set.
- Let the casserole rest on the counter for 5 minutes before slicing and serving.