2/3 cup water
1/3 cup coarse kasha (whole roasted buckwheat groats)
1 pound portabella mushrooms, stems discarded
1 cup finely chopped onion
1 cup finely chopped red bell pepper
3 tablespoons unsalted butter
2 garlic cloves, finely chopped
3 tablespoons finely chopped flat-leaf parsley
1 teaspoon soy sauce
1 1/2 cups fine dry bread crumbs, divided
1 large egg, lightly beaten
1/4 cup olive oil
1/2 cup mayonnaise
1 tablespoon Tabasco chipotle sauce, or to taste
8 large oval slices rye bread, cut into 4 1/2-inch rounds if desired, lightly toasted