Remove the stems and quarter the radicchio heads. Mix the balsamic, olive oil, chili flakes and chopped garlic to make a marinade. Submerge the radicchio, cut-side down, in the marinade for 1 hour prior to grilling.
Crumble the sausage into a medium saucepan and cook over medium-high heat until fully rendered and slightly browned, 8 to 12 minutes. Using a slotted spoon, remove the cooked sausage to a tray lined with paper towels.
Remove the radicchio from the marinade, reserving the marinade itself for dressing the finished radicchio. Grill the radicchio over an open flame starting with the cut-side down, and rotating until slightly charred on the outside (we suggest a charcoal grill, but if not using an outdoor grill, a griddle or grill pan would suffice). Once cooked, remove to a tray to cool. When the radicchio is cool, chop into 1-inch squares and toss with the reserved marinade.
Roll out the pizza dough into three 12-inch circles, 1/4-inch thick.
Start by grilling 1 pizza over medium-high heat on the grill. Once the bottom has a slight char, flip the pizza and top with some of the grilled radicchio, cooked sausage, chopped rosemary and taleggio slices (about 3 per pizza). Cover with the grill lid to melt the cheese and crisp the bottom, just a few minutes. Remove the pizza from the grill with a wide spatula to a cutting board. Let cool for a few minutes, and then slice into 6 or 8 pieces. Repeat with the remaining dough and ingredients.
Combine the flour, 3/4 cup water, yeast and sugar in the mixing bowl of a stand mixer. Knead on medium speed for 10 minutes. Place in an oiled bowl and allow the dough to rest for 1 hour, covered, in a draft-free spot.
Punch down and shape the dough into two to three 10-ounce balls. Allow to proof on an oiled tray or container covered with a towel or lid until almost doubled in size.