In a nonstick skillet, brown sausages, then cover with beer, reduce heat, cover and simmer until beer is almost disappeared.
Meanwhile, in another nonstick skillet, heat butter and oil and saute onions, thyme and garlic over low heat until caramelized, about 30-35 minutes; keeping the heat low and stirring often.
When caramelized, season to taste with salt and pepper.
Whisk cornstarch into stock until dissolved; stir into onions and stir over low heat, then bring a boil stirring, until mixture thickens.
Meanwhile, cook potatoes in boiling salted water 15-20 minutes or until fork tender; remove from heat and drain.
Add butter to the hot pan and allow to melt; add drained potatoes and mash; seasoning to taste with salt and pepper, adding in chives, and enough of the creme fraiche until it is as thin as you like.