In 12-inch skillet, heat oil over medium-high heat. Add pork; cook 10 to 12 minutes, turning once, until browned on all sides.
Spray 5- to 6-quart slow cooker with cooking spray. In cooker, place leeks. Top with pork. Sprinkle with salt and pepper.
Cover; cook on Low heat setting 7 to 9 hours.
Transfer pork to cutting board; keep warm. Stir mushrooms with liquid and leek soup mix into leeks. Increase heat setting to High. Cover; cook 3 to 5 minutes longer or until compote is thickened. (Stir in additional water for a thinner compote, if desired.)
Cut pork into 1/4-inch-thick slices; place on serving platter. Serve with compote.