Step 1Using a spice or coffee grinder, grind the annatto seeds, oregano, cumin, allspice and peppercorns to a powder.
Step 2In the bowl of a stand mixer fitted with the paddle attachment, or in a bowl using a hand mixer, beat the spices into the butter, along with the lime juice and zest. Cover and refrigerate up to 1 week; allow to soften before using.
Step 1Toast the chiles de Arbol: Heat a comal or dry skillet over medium-high heat. Add the chiles and toast, pressing down with a wooden spoon or spatula, until they begin to darken in spots and blister, about 1 minute per side. Remove from heat and set aside.
Step 2In a large, non-reactive container, whisk the salt into the water until dissolved. Whisk in the agave syrup, then the chiles, orange and pineapple juices, beer, avocado leaves, 1 bunch thyme and 2 halved garlic heads. Add the turkey, making sure it is completely submerged. Refrigerate the turkey for 2 days.
Step 3Remove the turkey from the brine and transfer to a rack. Air dry the turkey in the refrigerator for 24 hours.
Step 4Heat the oven to 350 degrees. Remove the turkey from the refrigerator and stuff the cavity with the remaining bunch thyme, oregano, lemon halves and remaining halved garlic head. Take half of the turkey butter and massage it onto the turkey meat under the skin.
Step 5Truss the turkey and place on a rack in a roasting pan. Roast the turkey until a thermometer inserted into the thickest part of the thigh meat reaches 150 degrees. Remove and brush with the remaining butter. Continue to roast, basting regularly, until the internal temperature reaches 160 degrees, 15 to 20 minutes. Remove and set aside for 30 minutes prior to carving.