In a large mixing bowl, combine all-purpose flour, baking powder, and salt. Cut in softened butter until the mixture resembles coarse crumbs. Firmly press this dough into a greased 9x13-inch baking dish.

Preheat your oven to 350°F (175°C). Bake the crust for 12-15 minutes or until lightly golden. Remove from the oven and let cool slightly.

In a skillet, melt a tablespoon of butter over medium heat. Add sliced onions and sauté for 10-15 minutes until golden brown. Set aside to cool.

In a large mixing bowl, combine cream cheese, sour cream, eggs, and cheddar cheese. Blend until smooth and creamy. Fold in the caramelized onions.

Pour the filling into the baked crust and spread evenly. Return to the oven and bake for an additional 25-30 minutes until set and lightly golden.

Remove from the oven and allow to cool for about 10-15 minutes before slicing into squares. Serve warm or at room temperature.