Mushroom-peppercorn sauce for chicken

1 tsp butter
½ medium onion, finely diced
1 tsp canola oil
8 large white mushrooms, sliced
1 tbs green peppercorns, drained
½ cup white wine
½ cup whipping cream
salt/pepper
In a large saucepan over medium-low heat, melt butter. Add onion and cook until translucent and soft. Add oil to a separate pan over high heat. Add mushrooms, season with salt and pepper, and sear for a few minutes, turning once, until they begin to brown. Transfer mushrooms to pan with onions. Add peppercorns and white wine; simmer until liquid is reduced by three quarters.
Into the mushroom and onion mixture, add cream. Increase heat to medium-high to bring back to a simmer, then reduce heat to medium. Simmer until sauce thickens enough to coat the back of a spoon. Serve over broiled chicken breasts.