Prepare the following, placing them all in a large bowl: Pick the leaves from 1/2 bunch parsley (about 1 1/2 cups). Pick the leaves from 1 bunch basil and tear into bite-size pieces (about 2 cups). Remove the leaves from 1 medium head radicchio and discard the core. Cut the leaves into 1-inch pieces (about 3 1/2 cups). Halve 2 medium Belgian endives lengthwise and remove the cores. Cut crosswise into 1/2-inch pieces and separate the leaves (about 4 cups).
Add 2 cups lightly packed arugula (about 2 ounces) and 2 cups lightly packed baby spinach (about 2 ounces). Toss with your hands until well-combined. If not using immediately, line an airtight container with a paper or clean kitchen towel, then transfer the spring mix into the container.
