Bring a large pot of salted water to a boil, add the tortellini, and cook according to package directions (about 5–8 minutes) until tender; drain and set aside.
While pasta cooks, heat 1 teaspoon olive oil in a large skillet over medium heat.
Add the sliced mushrooms and sauté until they are soft and lightly browned, about 4–6 minutes; remove the mushrooms from the pan and set aside.
Reduce heat to medium-low and pour the half and half into the same skillet. Add the crumbled feta, grated Parmesan, Italian seasoning, and fresh cracked pepper.
Whisk constantly over medium-low heat until the cheeses melt and the sauce thickens, about 7–10 minutes; watch carefully to prevent scorching.
Remove the skillet from the heat and add the cooked tortellini, sautéed mushrooms, and the baby spinach in handfuls; toss gently until the spinach wilts and everything is evenly coated in the sauce.
Serve immediately.
