Toss chicken pieces with berbere spice blend, cover, and marinate at least 30 minutes.
Heat niter kibbeh in a large skillet over high heat until shimmering.
Add chicken in a single layer, working in batches if needed. Sear 3–4 minutes per side until deeply browned and cooked through.
Remove chicken, set aside.
In the same skillet, reduce heat to medium, add onion, cook 4 minutes until softened.
Add garlic and jalapeño if using, cook 1 minute.
Return chicken to the skillet, toss with the onions and a sprig of rosemary, warm through 2 minutes.
Season with salt and serve hot.