Add olive oil, onion and garlic to a large pot.
Sauté the vegetables over a medium heat for a couple of minutes.
Then add celery, carrots, and potatoes, and continue to cook.
After about 5 minutes, add the lentils, mustard, soy sauce (or Worcestershire sauce if making a non-vegan version), brown sugar, and vegetable stock.
Ensure everything is fully mixed and then place a lid onto the pot.
Turn the heat up high and bring the mixture to a boil, then turn down to a simmer for about 30 minutes.
By now the potatoes will be soft, so mash some of them to thicken up the stew. Keep some for chunky potato tastiness.
Finally, add the frozen peas and allow them to heat through.
Taste your stew, and add salt if required.
Serve with crusty bread for a filling dinner.