MOVE oven rack to lowest position in bottom of oven. Heat oven to 375°F.
BEAT butter, brown sugar and salt in medium bowl with mixer on high speed until light and fluffy. Blend in flour and oats just until crumbly.
SPOON mincemeat into pie crust, spreading evenly. Sprinkle evenly with nuts, if desired. Finely crumble streusel mixture over filling.
BAKE on bottom rack 50 minutes or until crust is golden brown and filling begins to bubble. Cover with foil during last 20 minutes to avoid over-browning. Cool completely on wire rack.
Reconstitute 2 (9 oz.) packages condensed mincemeat according to package instructions. Prepare recipe as directed above, but substitute cooled, reconstituted mincemeat for 1 (27 oz.) jar of mincemeat.