Place top oven rack in center position and pre-heat oven to 425°F. Line a large, rimmed baking sheet with parchment paper or a Silpat™ baking sheet.
Divide the grapefruit in half and slice into equally thick sections, as shown. Arrange grapefruit slices on one side of the prepared baking sheet.
Slice the fennel bulb from top to bottom into equally thick slices, as shown. Arrange on the other half of the prepared baking sheet.
Sprinkle the sugar over the grapefruit slices, then drizzle the olive oil over fennel. Season the fennel with salt and black pepper, to taste. Tip: Use a pastry brush to evenly coat the fennel with the oil after drizzling.
Place baking sheet in the pre-heated oven and roast for 30-35 minutes, turning the fennel once halfway through, or until the fennel is golden brown and fork tender.
While the fennel and grapefruit are roasting, prepare the citrus vinaigrette by combining all the ingredients in a glass jar with a tight-fitting lid. Cover tightly and shake until well combined. Set aside.
Toss the avocado slices in a medium bowl with the fresh lemon juice to prevent browning and set aside.
When the fennel and grapefruit is done roasting, remove from oven and set aside to cool slightly.
Arrange the mixed salad greens on a large serving platter and top with the roasted fennel, grapefruit, and sliced avocado. Top with the pistachios and some chopped fennel fronds, if using. Serve immediately with a little citrus vinaigrette drizzled on top and the rest on the side.
Enjoy!