Scrub potatoes and cut in quarters. Place in saucepan with cream, milk, garlic, thyme and bay leaf. Bring to a boil over medium-high heat, reduce to a strong simmer and cook 15 minutes, until potatoes are just tender (the tip of a knife slides in with no resistance, but you need to give a nudge to get the potato to slide off again).
Strain potatoes, reserving cream. Discard bay leaf, thyme stems and garlic. Mash the potatoes (by hand, with a ricer, or a food mill). Add spoonfuls of the hot cream until the mixture is smooth and creamy. Add olive oil, butter and mustard. Season with salt and pepper, and serve hot.