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Sprinkle pork with salt and 1/4 teaspoon pepper. Heat 2 tablespoons oil in large Dutch oven over medium heat. Stir in pork; cook until lightly browned, about 5 minutes. Add water. Bring to a boil. Skim off any foam that rises during the first few minutes of boiling. Stir in pork stock, onion and garlic. Reduce heat; cover loosely and simmer until pork is tender, about 1 hour.
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While pork is simmering, line a baking sheet with foil. Arrange tomatillos on baking sheet. Broil 4 inches from heat source until blackened and soft on one side, about 5 minutes. Turn over; add peppers and garlic cloves to baking sheet. Broil until peppers and garlic have blackened, about 3 minutes. Turn peppers and garlic; continue to broil until all vegetables have blackened and are soft (2-3 minutes). Transfer tomatillos and any juices to blender or food processor. Puree until mixture is smooth; set aside.
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When pork is tender, remove meat from broth; set aside. Measure 4 cups of broth; set aside. Reserve remaining broth (about 1 1/2 cups).
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Wipe out Dutch oven. Add remaining 1 1/2 tablespoons oil. Heat over medium-high heat. When hot enough to make a drop of tomatillo puree sizzle, add puree all at once to hot oil. Cook until mixture thickens, stirring constantly (4-5 minutes). Stir in reserved 4 cups of broth and yams; bring to a boil. Reduce heat, cover loosely and simmer until yams are tender, stirring occasionally, about 20 minutes.
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In medium bowl, stir remaining 1 1/2 cups broth into masa. Gradually whisk masa mixture into simmering broth. Cook until thickened, stirring constantly (3-5 minutes). Stir in pork and navy beans. Simmer, covered, until thoroughly heated, stirring occasionally, about 10 minutes.