Preheat oven to 180C. Place potatoes in a roasting pan and roast until tender when pierced with a skewer (40-45 minutes). When cool enough to handle, peel potatoes and pass through a ricer into a large bowl. Add egg, flour and one garlic clove, season to taste and knead lightly until smooth. Shape mixture into quenelles with two lightly floured dessert spoons and place on trays lined with baking paper.
Cook raviules in a large saucepan of boiling salted water over high heat until they rise to the surface (1-2 minutes). Remove with a slotted spoon, drain well, set aside and keep warm.
Heat butter in a large frying pan over medium heat until nut brown (3-4 minutes). Add raviules and stir to coat, then sauté, turning occasionally, until golden brown (2-3 minutes). Add remaining garlic and chervil, stir to combine and serve with squab braised with juniper.