Add the potatoes, bay leaves, garlic, rosemary, and thyme to a large pot. Pour in the cream and milk, bring to a simmer over medium heat, and cook until the potatoes are fork-tender, about 20 minutes.
Drain the potatoes, reserving the cooking liquid, and discard the herbs and garlic. Press the potatoes through a potato ricer into a large bowl.
Add about 1½ cups of the reserved cooking liquid to the potatoes and stir until creamy. Add the butter, salt, and pepper, and stir to incorporate.
Garnish with thyme leaves, if desired.
Note: You can save any extra cooking liquid and use it to rehydrate leftover mashed potatoes the next day.