In a large Dutch oven, heat vegetable oil over medium-high heat. Season lamb with salt and pepper. Working in batches, cook lamb in a single layer until well browned on all sides, about 8 minutes per batch.
Return all of the lamb to Dutch oven, add garlic and onions, and cook until onions are soft, 6 minutes. Add flour and cook until it is well browned, 2 minutes.
Add wine, and cook for 2 to 3 minutes, scraping browned bits from the 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, 1 1/2 to 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 Dutch oven. (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, then drain. When onions are 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, turnips, and carrots to Dutch oven and simmer, covered, for 30 minutes. Add peas and simmer 10 minutes more, or until vegetables are tender.
Blanch beans until bright green and still crisp, about 5 minutes, then drain. Serve and scatter beans atop each serving.