Best Slow-Cooker Pulled Pork

Ingredients

• 1 yellow onion, finely chopped
• 3/4 c. ketchup
• 1/4 c. apple cider vinegar
• 3 tbsp. tomato paste
• 1 tsp. garlic powder
• 1 tsp. ground cumin
• 1 tsp. mustard powder
• 1 tsp. smoked paprika
• 1 (3- to 4-lb.) pork shoulder, trimmed
• Kosher salt
• Freshly ground black pepper
• Buns and coleslaw, for serving (optional)