For an indoor alternative that does not require a grill pan, you may cook the bacon in a heavy skillet and brown the Treviso in the bacon fat over medium-high heat.
Soak in cold water for 1 hour:
Have ready:
Drain the Treviso well and shake off as much water as possible. Prepare a hot grill fire or heat a grill pan over medium heat. Add the Treviso or radicchio to the grill or pan and grill briefly, just until charred on the cut sides and slightly wilted. Divide among plates and top generously with the sauce, bacon, if using, and: