Preheat oven to 425°. If you haven’t pre-seasoned your chicken, season the legs evenly with 3 teaspoons of kosher salt.
Heat a large sauté pan over medium-high heat and once it’s hot add the cooking oil. When oil is hot, place the chicken legs skin-side down and cook until golden brown, about 10 minutes. (Work in batches if pan is not large enough to hold all the chicken without crowding.)
While the chicken is searing, tear the bread into 1- to 2-inch pieces and place in a large bowl. Slice the radicchio in half from top to bottom, remove the core, tear the leaves into large bite-size pieces, and add to the bread bowl. Pluck the grapes into clusters of 5 to 10 grapes (loose individual grapes are okay, too) and add ⅔ of the grapes to the bowl. Toss to mix. Set aside.
When the chicken skin is crispy and golden brown, flip the legs to sear the flesh side for 1 minute. Once done, turn the heat on sauté pan down to low and place the legs skin-side up on a sheet pan. Set aside.
Add the onion, garlic, and rosemary to the sauté pan and cook, stirring occasionally, until the onion and garlic are fragrant and softened a bit but not colored, about 2 to 3 minutes. Turn the heat to medium and add the white wine, chicken stock, and olive oil. Stir in the mustard, bring to a simmer, and cook for 2 minutes.
Turn off the heat and pour the stock mixture over the torn bread and radicchio. With tongs or your hands, toss to evenly coat the panzanella. Once coated well, arrange the panzanella on the sheet pan in between the chicken legs. Keep the bread mostly in a single layer, evenly spacing the bread and the chicken to ensure it all cooks uniformly.
Once everything is arranged, top the panzanella with the rest of the grapes and place in the middle of the oven. After 10 minutes, rotate the sheet pan and cook for another 5 to 10 minutes. The chicken and panzanella are done when the bread is golden brown with crunchy bits and soft, mustard-saturated pieces underneath.
Serve hot with extra whole-grain mustard on the side.