Bring a large pot of salted water to a boil. Add penne pasta and cook al dente. Drain and reserve.
While pasta is boiling, heat olive oil and butter in a large sauce pot. Add onion and garlic and cook over low heat until fragrant. Add flour and stir to make a paste. Add milk 1 cup at a time, whisking between each addition until smooth. Add 8 oz cheese blend, nutmeg, dried basil, salt, and pepper. Simmer sauce over low heat for 10 minutes, stirring occasionally every few minutes. Remove from heat and stir in Chobani 2% Plain Greek yogurt.
Add pasta, spinach leaves and sundried tomatoes to white sauce and stir to coat evenly. Transfer to a large ovenproof dish and top with remaining 4 oz grated cheese. Bake at 375° degrees for 15 minutes and serve.