Cook pasta according to package directions until just tender - appx 9 minutes. Drain and set aside to cool.
In a large skillet, heat 2 Tbsp olive oil and add beef and crumble while browning. Add 3 cloves garlic, 1 Tbsp Italian seasoning, and a large pinch of salt. (If your Italian Seasoning has salt, omit the salt in this step.)
When beef is well browned, remove it to a large bowl. Stir in ricotta cheese, grated parmesan, spinach, and 2 cloves garlic. Salt and pepper to taste.
Heat oven to 350 and grease a 9x13 baking dish. Spoon 1-2 cups of marinara into the bottom of the dish.
Use a spoon to fill pasta shells using appx 2 tbsp filling and place them on marinara in the baking dish.
Dollop remaining marinara on top of shells and sprinkle on mozzarella cheese.
Bake uncovered at 350 for appx 30 minutes or until browned and bubbly.