Bring 2 cups water to boil in heavy medium saucepan over medium-high heat. Add tomatillos and jalapeño chile and cook until tomatillos are tender, about 7 minutes. Using slotted spoon, transfer tomatillos and chile to blender. Add 1/2 cup cooking liquid, cilantro and garlic. Pureé until sauce is almost smooth.
Heat oil in heavy large skillet over high heat. Add onion and sauté until beginning to soften, about 5 minutes. Add tomatillo sauce. Reduce heat to medium-low and simmer until sauce thickens, stirring occasionally, about 5 minutes. Mix in broth, then sugar; season sauce with salt and pepper.
Heat heavy large nonstick skillet over high heat. Add cheese and cook until slightly browned, about 1 minute per side. Bring tomatillo sauce to simmer. Add cheese to sauce and cook until cheese begins to melt, about 2 minutes.
Transfer cheese with sauce to shallow platter or divide cheese and sauce among 6 individual gratin dishes. Serve with tortillas.Makes 6 Source: