Preheat oven to 400 degrees. Slice one half of the fennel bulb into 1/4-inch slices and arrange in a single layer on a baking sheet. Drizzle the fennel slices with olive oil and sprinkle with salt and pepper. Roast for 15 minutes, flip, and roast for another 5-10 minutes. Remove from oven and set aside to cool.
Cut the peel and pith from the grapefruit, the navel orange, and the mineola orange and slice into 1/4-inch thick rounds, being sure to reserve the cut peels. Slice the remaining half of the fennel bulb into very thin slices. Add the roasted fennel, the raw fennel and the citrus rounds to a medium-sized bowl. In a small bowl, add the juice from 1/2 lemon as well as approximately 1/2 tablespoon of grapefruit juice, 1/2 tablespoon of navel orange juice and 1/2 tablespoon of mineola orange juice (extract this juice from the fruit remaining on the cut peels). Slowly drizzle in 1/4 cup extra virgin olive oil and whisk. Pour the dressing over the salad and add a pinch of salt and freshly cracked black pepper. Garnish with mint and reserved fennel fronds and gently mix before serving.