Put potatoes in a large pot of salted water over medium-high heat, and bring to a boil.
Reduce heat; simmer until tender, and easily pierced with a fork, about 15 minutes.
Drain, and put potatoes back into the pot.
Add warm half-and-half and butter and mash with a potato masher or electric mixer until creamy and smooth, adding more half-and-half if needed.
Stir in the miso, garlic, soy sauce, and lemon juice until well incorporated.
Garnish with chopped parsley or thyme, and serve immediately.
