It's not really a casserole, nor is it a stir fry. It's just what it is.
This is what I used/did but, as with most of my recipes, it's adaptable.
some sausages - I used four chipolatas (a YS find), each one cut into three bits
a small onion, sliced
50g frozen mixed peppers
1 mushroom, sliced
some passata (or chopped tomatoes - about half a can's worth)
mixed herbs, salt, pepper to taste
Using a frying pan, heat a little oil, drop in the sausages and fry, stirring until they are coloured and virtually cooked. It doesn't take long, Remove them with a slatted spoon and set aside.
Add a bit more oil, if necessary (it might not be) and saute the onions until softening and translucent. Add the peppers and the mushroom and continue frying for a short time.
Add the passata/chopped tomatoes and a squeeze of tomato puree, mixed herbs and seasonings to taste and return the sausages to the pan. Bring to a boil and very gently simmer for a while so that the sauce thickens a bit.
You can serve this with rice or pasta. I used rice.