Finely chop all of the vegetables and put in a medium bowl. Sprinkle cajun seasoning over the vegetables and then stir. Use a long knife to cut X shaped holes throughout the roast, on each side. Use your fingers to 'stuff' the seasoned vegetable mixture into the holes until all of them have some of the mixture. It is necessary to make sure that the vegetable mixture stuffed pretty far inside the roast so that each part of the roast is seasoned. Using a cast-iron pot would be best to prepare the roast.
Heat vegetable oil in the cast-iron pot. Really sear, without burning, each side of the roast so that each side is browned. If you have extra veggies you can saute them now with the meat, being careful not to burn them. Pour water over roast to cover it and then heat to boiling. Reduce heat a bit so that you continue to 'cook down' the meat and gravy over and over until you reach the desired tenderness of the roast and there is about 2-3 cups of liquid in the pot.
Once you've reached the desired tenderness, cut roast in slices on a plate. Add cold water to 3-4 tablespoons of cornstarch to make a slurry to add to the gravy to thicken it. Heat until thickened to your satisfaction. Put meat back into gravy and keep warm until you are ready to serve the meal. Traditionally this is served in Cajun country with rice, but you can also serve with mashed potatoes.