Step 1

Toast fennel seeds in a dry small skillet over medium heat, tossing often, until golden brown and starting to pop, about 3 minutes. Transfer to a spice mill or mortar and pestle and let cool; coarsely grind. Transfer to a small bowl and mix in oregano. Place pork on a wire rack set inside a rimmed baking sheet and rub with oil. Season all over with 1 Tbsp. salt, then rub with spice mixture, packing it on. Let sit at room temperature 2 hours or chill, uncovered, up to 2 days.

Step 2

Bring chiles, vinegar, butter, and sugar to a simmer in a small saucepan over medium-high heat; cook, stirring occasionally, until sugar is dissolved and sauce is reduced by about a third, about 5 minutes. Season with salt; cover and keep warm over low heat until ready to use.

Step 3

Prepare a grill for medium heat. Drizzle pork all over with oil, then grill, turning every 5 minutes or so, until extremely, gorgeously browned and crisp and an instant-read thermometer inserted into thickest part registers 120°, 35–45 minutes. Continue to grill, brushing with sauce and turning not quite every minute (you’re going to see flare-ups, but don’t worry), until pork is glazed and shiny and temperature on thermometer has risen to 130°, 5–10 minutes more. Transfer pork to a platter and let rest 30 minutes; reserve remaining sauce.

Step 4

Transfer pork to a cutting board and pour any accumulated juices on platter into reserved sauce. Return sauce to a simmer. Slice pork against the grain ¼" thick. Arrange on platter, spoon some sauce over, and season with salt. Serve remaining sauce alongside.