Heat oven to 350°. Combine duck, 1 ½ tbsp. salt, plus garlic, sliced onion, and cinnamon in a 6-qt. Dutch oven. Squeeze the orange quarters over the duck and add the spent oranges to the pot; toss to combine and arrange duck skin side up. Cook, covered, until duck is very tender, about 2 ½–3 hours. Let duck cool in the pot, then transfer to a cutting board. Shred meat into large chunks, discarding skin and bones. Strain pan drippings through a fine-mesh sieve into a measuring cup; chill until fat solidifies on the surface. Reserve ¼ cup fat; save remaining fat for another use.
Heat reserved duck fat in a 12” skillet over medium-high heat. Cook duck, stirring occasionally, until slightly crispy, 3–5 minutes. Season with salt; pile into warm tortillas and garnish with chopped onion, cilantro, and lime wedges. Serve with salsa, if you like.
