Place a large saucepan over medium-high heat and add oil.
Once the oil is ready, add the sausage slices and brown all sides.
Add garlic and onion, turn the heat to medium and cook until they turned soft.
Add tomato paste and cook for about 5 minutes; adjusting the heat to medium-low and mixing constantly.
Pour the stock.
Add oregano, thyme, chili flakes, salt, and pepper, then mix to combine.
Add the chopped bell pepper, carrots, celery, and potatoes, then mix.
Cover with a lid, turn the heat to medium-high, and bring to a boil.
Adjust the heat to medium-low and simmer until the vegetables and sausage are cooked.
Taste and adjust the seasoning.
Add cornstarch mixed with 2 tablespoons of water, and mix.
Put the lid back on, adjust heat to medium-high and bring to a quick boil.
Take the pan off the heat.
Transfer sausage stew into plates.