In small bowl, combine sliced jalapeño, 1 Tbsp lime juice, and a pinch each of salt and sugar.
Heat oil in medium saucepan on medium. Add onion and half of chopped jalapeño and cook, stirring occasionally, until tender and slightly golden, 4 to 5 minutes. Stir in garlic and cook 1 minute.
Stir in tomato paste, chili powder, cumin, and oregano and cook, stirring constantly, 1 minute.
Add tomato sauce, ½ cup water, and ¼ teaspooon each sugar, salt, and pepper and gently simmer, stirring occasionally, until thickened, 8 to 10 minutes.
Remove from heat, transfer to blender and puree until smooth. Transfer to large bowl and toss with chicken.
Meanwhile, in second small bowl, lightly mash avocados, remaining chopped jalapeño and 2 tablespoons lime juice and ¼ teaspoon salt. Spread on tostadas, top with chicken and sprinkle with cotija and pickled jalapeño.
