Combine the lentils, onion, green pepper, mushrooms, carrots, garlic powder, yellow mustard, maple syrup, crushed tomatoes, sea salt and black pepper in the slow cooker or Instant pot. Use a spatula to mix well.
If using the slow cooker: cover and cook on high for 4 hours or on low for 6 hours.
If using Instant Pot: put the lid on and set to “sealing” then press manual/pressure cooker and cook for 8 minutes on high pressure. Once finished, let the pressure release naturally for 5 minutes, then do a quick release.