Step 1

Preheat the oven to 400 degrees F.

Step 2

Heat the oil in a large frying pan over medium-high heat.

Step 3

Add the sausages and cook, turning regularly until brown on all sides, about 5-8 minutes.

Step 4

Transfer the sausages to a plate and slice into 3/4-inch-thick diagonal slices, about 4 slices per sausage.

Step 5

Reserve 1 tablespoon of the sausage oil in the pan and discard the excess.

Step 6

Add the onion, the garlic, the celery, and the thyme to the sausage oil in the pan and cook, stirring, until softened, about 4-5 minutes.

Step 7

Add the tomato sauce, the Worcestershire sauce, the stock, and the combined cornstarch and water.

Step 8

Stir to combine, cover, and bring to the boil.

Step 9

Reduce heat and simmer, uncovered, until the sauce has thickened, about 10 minutes.

Step 10

Stir in the sausages and the peas.

Step 11

Cook the sweet potatoes in a large saucepan of boiling water until tender, about 15 minutes. Drain the sweet potatoes and return to the saucepan over low heat.

Step 12

Add the butter and the milk to the sweet potatoes and mash until smooth.

Step 13

Stir in 3/4 of the cheese and season with salt and pepper.

Step 14

Spoon the sausage mixture into a 9x13-inch casserole dish and top with the mashed sweet potatoes, swirling the potatoes with the back of a spoon.

Step 15

Sprinkle the remaining cheese over the top.

Step 16

Bake until golden and bubbling, about 30-35 minutes.

Step 17

Serve.