Slice the onions. Peel the onions and cut them in half, root to stem. Use a mandoline to thinly slice them to about 1mm. If you're great with a knife, you can definitely do it by hand, but a mandoline will ensure that all the onion slices are even and that they'll cook evenly and melt into the sauce.
Render the pancetta. Heat a large (6-7 qt) dutch oven over medium-low heat and add the pancetta. Let it cook, stirring occasionally, until it's crispy and some of the fat has rendered. Use a slotted spoon to transfer the pancetta to a small bowl while leaving the fat in the pot.
Sear the beef. Increase the heat on your dutch oven to medium. Season your beef chunks liberally all over with salt and pepper. Sear for 3-4 minutes per side, until deeply browned. You may need to do this in batches. Once the beef is seared, transfer it to a plate.
Preheat an oven to 350 F.
Saute the veggies. Add the olive oil to the pot and add half of the sliced onion, the diced carrots, celery, and minced garlic. Cook, stirring occasionally, for 6-8 minutes, until the veggies are soft and starting to brown. Stir in the tomato paste and cook for another 2-3 minutes, until it's darkened in color.
Return the beef back to the pot. After the veggies are browned, stir in the crispy pancetta and half of the wine, using the liquid to scrape up any lingering browned bits on the bottom of the pot. Nestle the beef into the veggies and top it with the remaining raw sliced onions. Scatter in half of the cherry tomatoes with the bay leaf and sprinkle the onions with salt and pepper. Cover the pot and transfer to the oven with the lid just slightly cracked.
Braise. Let the beef and onions braise for about 2 1/2 hours, checking on it occasionally to make sure the liquid hasn't totally evaporated. If it looks dry or too dark, stir in a little water or broth. Remove the pot from the oven and let it rest for 10 minutes. Remove the lid and place the pot on a burner over medium heat and stir in the rest of the tomatoes and wine. Simmer, stirring occasionally, until the tomatoes have blistered and the beef is totally shredded with the onions melted into the sauce, about 15 minutes.