Heat 2 tablespoons of the oil in a large skillet over medium heat.

Add garlic and stir until fragrant, about 30 seconds.

Add mushrooms and cook, stirring, until they start to brown and release their liquid, 3 to 5 minutes.

Add spinach and cook until just wilted, 2 to 3 minutes.

Season with 1/4 teaspoon of the salt and 1/4 teaspoon of the pepper and stir in pine nuts. Remove from heat and let cool.

Preheat the oven to 350°F. Season pork all over with remaining 1/2 teaspoon salt and 1/4 teaspoon pepper.

Spread cooled spinach-mushroom mixture evenly over pork, leaving a border around the edges.

Roll up or fold in half like a book and tie pork at 1-inch intervals with kitchen twine.

Heat remaining 2 tablespoons oil in a large skillet over medium-high heat. Add pork, seam side down, and brown for 3 to 4 minutes, then brown the remaining sides and ends.

Transfer pork to a roasting pan and roast until an instant-read thermometer inserted into thickest part reads 150°F, 1 to 1 1/4 hours. Let pork rest for 10 minutes, then cut into thick slices and serve.