Preheat oven to 400 degrees.
Fit a large rimmed sheet pan with aluminum foil.
Toss the tomatillos, serrano, and onion with olive oil,salt, and pepper.
Roast for 25-30 minutes, or until lightly charred and completely softened.
Let cool slightly, and remove seeds from serrano.
Place the roasted vegetables in a blender with lime juice,oregano, cumin, paprika, and cayenne.
Process until smooth.
Pour the mixture into a medium saucepan, and add in chickenbroth.
Cook over medium heat until the mixture comes to a lightboil, reduce heat to low, and simmer for 10 minutes, stirring occasionally.
Remove from heat, let cool completely, and pour into a jar.
Refrigerate mixture for up to 5 days.