To make chimichurri: Process all ingredients together in blender, occasionally stopping to scrape down sides, until smooth. Transfer chimichurri to a serving bowl, set aside (makes about 1/2 cup chimichurri). 

To make pork: Butterfly pork, cutting tenderloin lengthwise, stopping about a quarter inch from the bottom. Open pork, make a lengthwise cut on each meaty side, and open again so pork is about 5 inches wide. 

Heat 1 teaspoon oil in medium nonstick skillet over medium heat. Add shallot and garlic and cook until shallot softens, about 1 minute. A handful at a time, stir in spinach, letting each batch wilt. Cook, stirring occasionally, until spinach is tender, about 3 minutes. Drain in sieve and let cool until easy to handle. Press hard on spinach mixture with spoon to remove excess liquid. Transfer to bowl and stir in bell pepper, cheese, breadcrumbs, 1/4 teaspoon of salt and 1/8 teaspoon of pepper. Spread filling on pork. Roll up lengthwise and tie crosswise with kitchen twine (or unwaxed plain dental floss) in three or four places. Brush with remaining teaspoon oil and season with remaining salt and pepper. 

Prepare outdoor grill for indirect and direct cooking over medium-high heat (about 400 F.). Grill tenderloin over direct heat, with the lid closed, turning occasionally, until browned, 5 to 7 minutes. Move to indirect heat and continue grilling, with the lid closed, until the internal temperature is 145 F (medium rare) to 160 F (medium) on a digital meat thermometer, 17 to 22 minutes more. Remove from heat and let rest for 3 minutes. 

Remove twine. Cut tenderloin crosswise into 1/2-inch slices. Serve with chimichurri on side. Leftover chimichurri can be covered and refrigerated for up to 2 days.