Step 1: Make the dressing: In a bowl or jar, whisk or shake together the olive oil, lemon juice, red wine vinegar, Dijon mustard, minced garlic, red pepper flakes, salt, and pepper until well combined and slightly emulsified.
Step 2: Marinate the beans: Pour canned cannellini beans into a colander and rinse well under cold water. Let them drain completely. In a small mixing bowl, pour half the salad dressing over the beans and gently stir. Reserve the other half to dress the salad. Cover and let rest for at least 15 minutes, or refrigerate overnight and make the salad the next day for maximum flavor in the beans. The longer it sits, the more the flavors permeate the beans. Proceed with the remaining steps once your beans have marinated to your liking.
Step 3: Soak the shallot or red onion in ice water: Place the sliced shallot (or red onion) in a small bowl of ice water for about 5 minutes. This quick step mellows the sharp flavor while maintaining crispness. Drain well before using.
Step 4: Prep the remaining ingredients: For the fennel, trim off the stalks and fronds, reserving about 2 tablespoons of the feathery fronds. Peel away any tough outer layers, then slice the bulb in half lengthwise and remove the triangular core from each half. Place the halves cut-side down and slice crosswise into thin half-moons, about ⅛-inch thick. Coarsely chop the reserved fronds. Next, prepare the other herbs: roughly chop the parsley and thinly slice the basil. Finally, roughly chop the pistachios.
Step 5: Assemble: Add the salad greens, beans, veggies, and herbs to a big bowl. Pour the vinaigrette over the top and fold gently to combine, being careful not to mash the beans. Top with feta cheese and chopped pistachios.
Step 6: Taste & adjust: Before serving, add more salt, more red pepper, a splash of vinegar, or a drizzle of olive oil if needed.