Heat a large skillet over medium-high heat; place tomatoes, tomatillos, onion, and garlic into the hot skillet and stir until vegetables are lightly charred, about 20 minutes. Remove vegetables from skillet; stir sesame and pumpkin seeds into the hot skillet, stirring until seeds are lightly toasted, 2 to 3 minutes. Remove from heat.

Toast baguette slice in a toaster. Place baguette slice, tomatoes, onion, garlic, sesame seeds, cilantro, salt, and black peppercorns into a blender; pulse several times to chop, then blend the mixture until smooth. Pour 3 cups chicken broth into blender and puree again.

Strain blended sauce through a sieve and pour into a large saucepan; stir olive oil into sauce. Place over medium-high heat, bring to a boil, and cook for 3 minutes. Stir chayote, potatoes, and corn into sauce; stir in 3 cups chicken broth. Bring back to a boil, reduce heat to low, and simmer until chayote and potatoes are tender, about 20 minutes.