Bring a large pan over high heat.  Add oil and onion, saute a few minutes.  Cut sausage links into 4 even pieces about 2 inches long.  Add to pan and let brown, stirring with a spoon as needed, not too often you want it to brown.  After about 8 minutes add mushrooms and garlic and saute until mushrooms begin to brown.  Then add salt and pepper to taste and ketchup, just a little bit brings the whole thing together.

Serve with rice or potatoes or on a bun.