Cook 4 bacon slices in a skillet with 1 teaspoon vegetable oil until crisp, 8 minutes. Drain on paper towels, then crumble. Add 1 sliced red onion to the drippings, season with salt and pepper and cook 2 minutes; let cool. Combine a 14-ounce bag coleslaw mix, 3/4 cup each sliced radishes and halved cherry tomatoes, 1/2 chopped cucumber and the red onion in a bowl. Puree 1 avocado, 3/4 cup buttermilk, 1/2 cup fresh parsley, 2 tablespoons chopped chives, the juice of 1/2 lemon, 1 teaspoon kosher salt and some pepper; toss with the slaw. Add the bacon. Chill 30 minutes.