Slow Cooker Coconut Lentil Curry

2 cups dried green lentils (you can also use red or brown)

3 cups vegetable or chicken broth (low sodium or regular)

2 15-ounce cans fire-roasted crushed tomatoes

1 medium onion, finely chopped

3 cloves garlic, minced

1 tablespoon garam masala powder or standard curry powder 

2 teaspoons ground turmeric

2 teaspoons salt

1/2 teaspoon  black pepper

1 (14-ounce) can unsweetened full-fat coconut milk

2 cups mixed frozen vegetables