Step 1: Caramelize the onions. In a large, heavy-bottomed skillet, melt butter and olive oil over medium-low heat. Add sliced onions and a pinch of salt. Cook, stirring occasionally, for 30-45 minutes until deeply golden brown, soft, and sweet. Set aside to cool slightly.
Step 2: Thoroughly drain the sauerkraut. Press out as much liquid as possible using a colander and the back of a spoon or your hands.
Step 3: Preheat oven to 375°F (190°C). Line a baking sheet with parchment paper.
Step 4: On a clean work surface, lay out a large flour tortilla. Spread a generous, even layer of caramelized onions over the tortilla, leaving about a half-inch border.
Step 5: Layer about 2 ounces of pastrami evenly over the caramelized onions on the tortilla.
Step 6: Distribute about 2 tablespoons of drained sauerkraut evenly over the pastrami.
Step 7: If using, place half a slice of Swiss cheese or sprinkle about 1 tablespoon of shredded Swiss cheese over the sauerkraut.
Step 8: Carefully but firmly roll up the tortilla from one side to the other, creating a tight log. Ensure the filling is compact.
Step 9: Slice each rolled tortilla into 1 to 1.5-inch thick pieces. Arrange the individual rolls seam-side down on the prepared baking sheet.
Step 10: Repeat steps 4-9 with the remaining tortillas and filling.
Step 11: Lightly brush the tops of the rolls with melted butter or beaten egg for a golden finish (optional).
Step 12: Bake for 15-20 minutes, or until the tortillas are golden brown and crispy, and the filling is heated through and cheese is melted and bubbly. Serve hot.