Step 1

In large pot of salted boiling water, cook pasta al dente, approximately 4 minutes. Drain and set aside.

Step 2

To make pesto, place the spinach, garlic and cheese into the bowl of a food processor and pulse several times. While the food processor is running, slowly add the olive oil in a steady small stream. Adding the olive oil slowly, while the processor is running, will help it emulsify and help keep the olive oil from separating. Occasionally stop to scrape down the sides of the food processor. Season with salt and pepper.

Step 3

Preheat oven to 375F.

Step 4

In a 10 inch cast iron pan or oven safe pan, melt 1/3 cup of butter over medium heat. Whisk in flour and continue whisking until mixture has turned golden. Whisk in the milk and remove any lumps. Cook until slightly thicken, about 3-4 minutes. Add in mustard and salt and paper. Whisk until smooth and slightly thicken. Add grated cheese and continue whisking until cheese has melted and mixture is creamy.

Step 5

Add pasta shells to cheese mixture and mix well.

Step 6

Mix in pesto sauce, green onions and feta cheese.

Step 7

Pour pasta into cheese sauce and stir until well blended.

Step 8

Mix together remaining 2 tbsp of melted butter with breadcrumbs and parmigiano cheese. Sprinkle evenly on top of pasta.

Step 9

Place in oven and bake for 20-25 minutes until cheese is bubbly and top is golden. Serve hot!