Preheat the oven to 190°C, Gas Mark 5.
Using a sharp knife, slice the fennel, the pear and the onion thinly.
Layer them into the bottom of an oven proof dish.
Pour over the stock and vodka. The stock will add salt, and the sausages are seasoned, so you don't need to add salt. Grind a little white or black pepper over if you fancy. 
Add the sausages.
Pop into the oven for 25 minutes or so, turning half way through if you want to brown them all over. (They'll cook fine without you faffing over them). 
Remove and allow them to rest for five minutes.
Serve with gravy and mash, chips and peas, or new potatoes and veggies.