Sweet-potato-and-lentil-topped mini cottage pies

Ingredients

• 1 medium sweet potato, peeled and cut into 1-in. chunks

• 1 cup dried red lentils

• 3 tbsp butter, divided

• 1 tsp salt, divided

• 1 onion, finely chopped

• 1 celery stalk, finely chopped

• 450 g lean ground beef

• 2 tbsp tomato paste

• 1 tbsp all-purpose flour

• 1 1/2 cups low-sodium beef broth

• 1 tbsp Worcestershire sauce

• 1 cup frozen green peas