Spinach egg noodle casserole: rachael ray again

Ingredients

• 2 10 oz packages of frozen spinach
• 1 lb extra wide egg noodles
• 1 Tbsp olive oil
• 1 medium onion
• 2 cloves garlic
• 3 Tbsp butter
• 2 rounded Tbsp flour
• Salt & pepper to taste
• ¼ tspn dry mustard
• 2 cups whole milk
• freshly grated nutmeg
• 2 egg yolks
• 1 cup panko breadcrumbs
• 1 cup gruyere cheese
• ½ cup parmigiano-reggiano cheese