Heat olive oil and butter in a large skillet over medium heat. Add the sliced mushrooms and let them cook undisturbed for a few minutes. This helps them brown properly and develop a deep, savory flavor.
Once the mushrooms are golden, stir in the minced garlic, salt, pepper, and red pepper flakes. Cook briefly until the garlic becomes fragrant. Be careful not to burn it, as garlic can turn bitter quickly.
Pour the dry orzo directly into the skillet and stir it with the mushrooms. Let it toast for about a minute. This step adds a subtle nutty flavor and helps the pasta hold its texture.
Slowly pour in the warm vegetable broth and bring everything to a gentle simmer. Stir occasionally and let the orzo cook until tender, absorbing most of the liquid. This usually takes about 8 to 10 minutes.
Lower the heat and stir in the heavy cream and grated Parmesan cheese. The sauce will thicken naturally as the cheese melts. Keep stirring until everything looks smooth and glossy.
Fold in the fresh spinach and let it wilt into the warm orzo. This only takes a minute or two. The spinach should stay vibrant green and tender.
Taste the orzo and adjust seasoning if needed. Turn off the heat and let it rest for a couple of minutes before serving. This helps the sauce settle into a perfect creamy consistency.