The flavor of this classic French stew improves when it's refrigerated overnight.
In a large flameproof casserole, heat vegetable oil over medium-high heat. Season lamb with salt and pepper, and place in casserole. Brown meat well on all sides.
Add garlic and diced onions, and cook until onions are soft, 5 to 10 minutes. Add flour, and cook until it is well browned, 3 to 4 minutes.
Add wine and cook for 2 to 3 minutes, scraping bottom and sides of pan. Add stock, tomatoes, rosemary, thyme, and bay leaf. Bring to a boil, then reduce heat to low; cover, and cook until meat is tender, about 1 to 1 1/2 hours.
Remove meat; place in a large bowl and set aside. Reduce sauce over high heat until thickened, about 10 minutes. Adjust seasonings. Skim off any fat. Strain sauce over meat, discarding solids; return meat and sauce to casserole. (The navarin can be made ahead up to this point. Cover and refrigerate for up to 2 days. Reheat and continue as directed.)
Bring a small saucepan of water to a boil, and add pearl onions. Boil 2 minutes, and drain. When cool enough to handle, peel and set aside. Peel baby carrots; trim tops to 1/4 inch. Repeat with baby turnips. If using large carrots and turnips, peel and cut into 1/2-inch pieces. Add onions and carrots to casserole and simmer, covered, for 5 minutes. Add turnips and peas, and simmer 5 minutes more, or until vegetables are tender.
Blanch beans until bright green and still crisp, then drain. Serve stew over mashed potatoes, and scatter beans atop each serving. Sprinkle with chervil, and serve immediately.