Slice the duck breasts into thin strips of about 1/3-in (1 cm) and lay them on a plate. Drizzle with oil and season with chili powder and ground cumin.
Preheat a large non-stick pan over medium-high heat. When hot, sauté the duck strips for 4 to 6 minutes, stirring constantly. Halfway through cooking, add the onion and peppers and continue to sauté while stirring.
Remove the pan from the heat and drizzle with lime juice.
Transfer the duck mixture to a serving bowl and serve immediately at the table with the avocado slices, chimichurri salsa, grated cheddar cheese and tortillas. Allow each guest to make their own fajitas as they like.
Place all ingredients in a bowl and season generously with salt and pepper.
Add a few pinches of hot pepper flakes and mix.
Cover and let stand for 1 hour in the refrigerator before serving.
