- Place chicken in a medium saucepan; cover with water and bring to a boil over high heat. Reduce heat to medium-low, cover, and let simmer until chicken is cooked through, 8-12 minutes. Remove chicken from the saucepan and place on a plate; discard cooking liquid.
- Quarter the jalapeño peppers lengthwise; seed and remove ribs of the jalapeños with a spoon. Finely dice and set aside.
- Heat a small saucepan over medium heat. Peel and small dice onion. Peel and mince garlic.
- Once the saucepan is hot, add oil and swirl to coat the bottom.
- Add the jalapeños, onion, garlic, chili powder, and salt to the saucepan. Cook, stirring occasionally, until vegetables have softened, 4-5 minutes.
- Transfer cooked vegetables from the saucepan to a medium bowl. Shred the chicken and add to the bowl with the vegetables. Stir to combine and set aside.
- Return small saucepan to stove over medium heat. Melt the butter, then add the flour. Whisk for 1 minute.
- Add the broth and salt to the saucepan; whisk until thickened, 2-3 minutes. Remove from the heat and let cool, 3-5 minutes.
- Preheat oven to broil.
- Grease the bottom of a baking dish with oil. Place ⅓ cup of chicken filling in the center of each tortilla. Top chicken with 1 tbsp of cheese. Roll tightly and place seam-side down in the baking dish.
- Add yogurt to the pan with the sauce and whisk until smooth. Pour the sauce over the tortillas in the baking dish. Top with the remaining cheese. Place baking dish in the oven and broil until cheese is melted and bubbly, 3-4 minutes.
- Divide the enchiladas between plates and top with tomato and green onion.