⅓ cup raw, shelled pepitas (pumpkin seeds)
¼ cup sesame seeds
Salt and freshly ground black pepper
2 tablespoon vegetable oil
1 onion, chopped fine
1 fresh jalapeno chili, stemmed, seeded, and chopped fine
3 garlic cloves, minced
¼ teaspoon dried thyme
¼ teaspoon dried marjoram
6 ounces tomatillos, husks removed, stems removed, rinsed well and dried, chopped
1 ½ cups chicken broth
4 boneless, skinless chicken breasts trimmed
1 cup fresh cilantro leaves
1 tablespoon lime juice
Pinch of sugar (or ½ teaspoon honey or agave)
Toast the pepitas and sesame seeds in a 12-inch nonstick skillet over medium heat until the seeds are golden and fragrant. When the first pumpkin seed pops, stir constantly until all have popped from flat to round, about 5 minutes. Do not let them darken past golden (usually caused by too high a heat or not enough stirring) or the sauce will be brownish and slightly bitter. Transfer them to a bowl. Set aside 1 tablespoon for garnish.
Heat oil in the now empty skillet. Add the onion and ½ teaspoon salt to the skillet and cook over medium-high heat until softened, 5 to 7 minutes. Stir in the jalapeno, garlic, thyme, and marjoram; cook until fragrant, about 30 seconds. Stir in the tomatillos, broth, and toasted seeds, cover; cook until tomatillos begin to soften, about 10 minutes.
Season chicken breast with salt and pepper. Nestle the chicken into the skillet. Cover, reduce heat to medium-low, and cook until the chicken registers 160°F, 10 to 15 minutes. Flip chicken halfway through. Transfer chicken to a platter and tent with aluminum foil, and let rest for 5 to 10 minutes.
Carefully transfer mixture left in the skillet to blender. Add cilantro leaves, lime juice, and sugar, and process until almost smooth, about 1 minute. Season the sauce with salt and pepper to taste. Serve the chicken with the sauce, sprinkling individual portions with reserved toasted seeds and minced cilantro.