Add potatoes, cream, milk, garlic, thyme and bay leaf into a large saucepan and gently boil for 15 minutes until potatoes are just tender and the tip of a paring knife goes through with little resistance. Strain potatoes and reserve the cream mixture but discard the bay leaf, thyme stems and the garlic. Mash the potatoes and fold in the cream mixture until the potatoes are smooth and creamy. Fold in about a tablespoon of the olive oil, the butter, and the grainy mustard. Season with salt and pepper.