Toss the celery and onion in a rectangle roasting pan with the olive oil, salt and pepper. Place in the ovento get about a 10 minute head start on cooking while you prepare the other ingredients.
Crush some grapes with the back of a wooden spoon (see notes) to help them release their juices while cooking, or cut in half. Add the grapesto the roasting pan, including some on the stems, for visual effect.
Pierce the sausages a few times with a toothpick to allow steam to escape, then lay themon the grapes and nestle the bay leaves closely around the sausages.
Sprinkle with thyme, including a few whole stems, and a dash of salt and pepper. Return the pan to the oven.
Roast uncovered for 25 minutes, then turn the sausages to brown on the other side for another 25 minutes. (The minimum cooked temperature of the sausage should be 160°F/71.1°C) If sausages are not golden to your liking, turn the oven on broil for a few minutes until they achieve a deep golden color. (Carefully watch them the whole time you have the oven on broil—they can go from beautiful to burnt fast.)
To serve, garnish each plate with a bay leaf, stems of thyme, and a stem of grapes from the roasting pan.
Preparing Grapes: How many you crush, or halve, depends on the kind of grapes you are using. If using firm grapes, such as most oval varieties, crush around ¼ of them. If using juicy grapes, such as the large Globe variety, you need only crush a few, or leave them all whole.Best Way to Crush Grapes - I've found the best way is on a cutting board with the back of a wooden spoon. With the palm of your hand on top of the spoon, gently press down on each grape in a front to back motion. Make sure to angle the spoon away from you to protect from grape splatters. Reheating: This dish is excellent reheated in the oven the next day. Bake uncovered at 375°F (190°C) in its juices. If needed, crush or add a few grapes, or a dash of wine.