Adjust an oven rack to the middle position and heat the oven to 350 degrees.
Cook the pancetta and ⅔ cup of water in a Dutch oven over medium-high heat, stirring occasionally, until the water has evaporated and a dark fond forms on the bottom of the pot, 8 to 10 minutes.
Add the onion and fennel bulb and cook, stirring occasionally, until the vegetables soften and start to brown, 5 to 7 minutes.
Stir in the garlic, salt, thyme, and pepper and cook until fragrant, about 30 seconds.