Place a large pot over medium high heat. Add olive oil, butter, and chopped onion. Cook for 3 to 4 minutes, stirring often, until softened and translucent.
Stir in minced garlic and Italian seasoning. Cook for about 30 seconds until fragrant, being careful not to brown the garlic.
Add roasted petite diced tomatoes with juices, lemon juice, and Dijon mustard. Stir well and cook for 1 minute to blend flavors.
Stir in uncooked orzo and toast lightly for 2 to 3 minutes, stirring frequently.
Pour in vegetable broth and room temperature heavy cream. Bring to a gentle simmer, then reduce heat to medium low. Simmer uncovered for about 10 minutes, stirring every few minutes, until orzo is tender and sauce is creamy.
Remove from heat. Stir in Parmesan cheese, baby spinach, and chopped parsley until spinach wilts and cheese melts smoothly.
Cover and let rest for 2 to 3 minutes to thicken. Season with salt and black pepper to taste, then serve warm.