Heat olive oil and butter in a skillet over medium heat until fragrant. Add finely chopped onion and sauté for 2-3 minutes until softened. Toss in minced garlic and cook for 1 minute. Add orzo pasta and stir to coat. Pour in broth and bring to a simmer, then reduce heat. Cook for 5 minutes, stirring occasionally, until orzo starts to absorb moisture. Add heavy cream and continue cooking for 5-7 minutes until orzo is al dente and mixture is creamy. Fold in cooked shrimp, Parmesan, and red pepper flakes (if using). Stir in lemon zest and lemon juice. Toss with spinach leaves until just wilted. Season with salt and pepper. Garnish with parsley before serving.