Preheat oven to 450°F.
I have a very specific way that I like to cut cauliflower (NO FLORETS!) so pay attention here. First, trim off the leaves and the very bottom of the cauliflower stem. Now slice it in half, through the stem. Place the two halves cut-side down on a cutting board, then slice it into 1/2-inch thick pieces. Cut or pull apart the individual florets. Place the cauliflower on half of the sheet pan.
Trim the stems off of the fennel bulb, cut it in half through the stem, then slice it into 1/4-inch thick slices and place them on the other half of the sheet pan. Drizzle the cauliflower and fennel with 3 tablespoons olive oil, then season with 1 1/2 teaspoons kosher salt and 1/4 teaspoon black pepper. Toss to coat and spread into an even layer.
Roast for 25 minutes on the center rack in the oven. The cauliflower and fennel should be tender, and just beginning to brown around the edges. If they’re not, roast for another 5 to 10 minutes before continuing to the next step.
While the vegetables roast, season 2 to 4 boneless skinless chicken breasts with a thin layer of olive oil, 1/4 teaspoon kosher salt and a sprinkle of pepper per breast. Set aside to come to room temperature while the vegetables cook.
Make the green sauce by blending 1 cup fresh cilantro leaves, 1/3 cup toasted cashews, 1/3 cup olive oil, the juice of 2 limes, 1 garlic clove, 1 to 2 tablespoons verde hot sauce (depending how hot you like things), and 1/4 teaspoon kosher salt until smooth. If it’s too thick (it should be drizzle-able) or your blender is having a hard time blending it, blend in 1 tablespoon of water at a time. Taste and adjust seasoning to taste (always adjust seasoning for YOUR tastebuds!). Too tangy? Add another splash of oil. Need more flavor? Another pinch of salt. Love spice? Add more hot sauce! Pour the sauce into a bowl.
After 25 minutes, add the chicken breasts to the pan, on top of the vegetables is fine if there’s no space. No, you don’t need to stir the cauliflower or fennel! Roast the chicken for 12 minutes, then check the internal temperature using a meat thermometer. If it’s not finished, continue cooking, checking the temperature frequently, until the chicken reaches 160°F.
Chicken must be 165°F to be safely served, but I like to pull chicken breasts out at 160°F as they will continue cooking even once removed from the oven, and have so little fat that we don’t want to risk overcooking them
Transfer the chicken breasts to a cutting board.
Stir 2 cups fresh spinach into the vegetables. Taste and adjust seasoning with more salt and pepper as needed.
Slice the chicken breasts on the bias (see photo - don’t slice straight up and down, slice at an angle, this will make the chicken more tender). Serve the chicken alongside the fennel and cauliflower, with lots of sauce!