Radicchio with caramelized onions and sausage

I adapted this dish from a recipe Peggy provided with the week 1 CSA. My version used the 'Sugarloaf' radicchio, the one that looks sort of like Romaine lettuce and that I'm not sure is really a radicchio (I think it's just a chicory). As we've discussed before, radicchio is bitter, it's supposed to be, but this dish really balances that bitterness with the sweetness of well-caramelized onions and the savoriness of sausage. The lingering crunchy texture of the radicchio is particularly satisfying against the textures of the onion and sausage. I suspect there might not be as much crunch if you're using the more delicate 'Bel Fiore' radicchio.

Radicchio with caramelized onions and sausage

Adapted from here.
Ingredients

• 1 tbsp. butter
• 2 tsp. oil, divided
• 2 medium onions, red or yellow, halved and thinly sliced
• A few pinches of salt
• A few pinches of sugar
• 1 large head radicchio, cored, thinly sliced
• 2 tbsp. balsamic vinegar (this is what I'm currently using, available at Jungle Jim's) 
• 3 chicken sausages, casing removed and cooked (I used two Greenacres Tomato Basil and 1 Greenacres Sweet Italian, just because that's what I happened to have in the freezer)

Directions

Melt butter with 1 tsp. oil in a large skillet over medium heat. Add onions plus a few pinches of salt and sugar and cook until caramelized (soft and brown), stirring often, about 25 minutes. Add the radicchio and saute until wilted, a few minutes. Add the balsamic vinegar and cook over medium-high heat until the liquid is mostly absorbed (but be sure the radicchio retains some texture!). Season to taste with salt and pepper.

Meanwhile, remove the casings from the sausages and cook them in 1 tsp. oil in a skillet, breaking up the meat until no pink remains. 

Mix the cooked sausage into the radicchio/onions and serve.