Preheat oven to 325°. Coarsely grind fennel seeds and red pepper flakes in a spice mill or with mortar and pestle.
Place chicken on a rimmed baking sheet and rub all over with oil; season generously inside and outside with salt. Sprinkle chicken all over with ground spices. Arrange breast side up and roast until an instant-read thermometer inserted into the thickest part of breast registers 155°, 70–80 minutes. Let cool.
Transfer chicken to a cutting board. Carve or remove meat from bones and shred; discard bones.