• Preheat the oven to 180C/350F and lightly grease a rectangular oven pan (lasagne dishes work well).
• Cook the gf penne in a large saucepan of salted boiling water following the directions on the box to achieve 'al dente'. Don't overcook it! Drain and set aside.

Gluten-Free White Sauce

• Melt butter in a saucepan over a medium heat. Add the gluten-free flour and stir to a smooth paste.
• Slowly add the milk and keep stirring until the mixture boils and thickens.
• Remove the white sauce from heat and gently mix in the mustard. Set aside to cool slightly.
  

Tuna Casserole Mix

• Whisk the eggs and sour cream together in a large bowl.
• Add the gf white sauce mixture and stir together until well combined.
• Mix in the cup of cheese and the drained tuna. Season with salt and pepper to taste.
• Pour in the cooked pasta and stir to combine.
• Spread the mixture into the prepared pan and smooth the surface over.
  

Breadcrumb Topping

• Melt the 20g(1 oz) butter in the microwave, then add the 1/2 cup cheese and breadcrumbs and mix together.
• Scatter evenly over the tuna casserole surface.
• Bake in the oven for 30 minutes, or until set and golden on top.