Preheat the oven to 375°F.
Butterfly the flank steak by slicing it horizontally and then pounding it flat.
Season both sides with salt and pepper and set aside.
Heat 2 tablespoons of oil in a small saucepan over medium-high heat.
Add the chopped onion and cook for 2-3 minutes until tender and beginning to turn golden.
Add the garlic and sauté for an additional minute.
Remove from heat and stir in the cream cheese until fully combined.
Season the mixture with salt and pepper to taste.
Spread the cream cheese mixture evenly over the prepared steak.
Layer the baby spinach and sliced cheese on top.
Roll the steak up tightly and secure it with kitchen twine, toothpicks, or short kabob skewers.
Using a sharp knife, slice the roll between the ties or toothpicks to create pinwheels.
Lightly season the pinwheels with salt and black pepper.
Heat a large, heavy-duty pan over medium-high heat.
Add the steak pinwheels and sear them for 2-3 minutes on each side.
Transfer the pinwheels to a baking dish or oven-safe pan.
Bake for 10 minutes.
Remove from the oven and tent the pinwheels with aluminum foil.
Let them rest for 5 minutes before serving.