*Preparing lentil purée: Add 2 ½ times the amount of water as lentils. Bring to a boil, reduce heat, cover and simmer for 45 minutes. Stir frequently. Cool slightly and do not drain. In small batches, mash or pureè lentils with a sieve, food mill, blender, food processor or potato masher. Purée should be the consistency of canned pumpkin. Add water to thin if needed.
Preheat oven to 375°F. Lightly grease baking sheets.
In a large mixing bowl, cream together butter, brown sugar, and white sugar. Add eggs and vanilla; mix well. Add puréed and cooked lentils, and mix in evenly.
In a separate bowl, mix together dry ingredients.
Add dry ingredients to wet ingredients a little at a time, alternating with butterscotch chips and walnuts, until mixed evenly.
Drop by rounded tablespoons onto baking sheet, decorate with butterscotch chips, and bake until pale golden, 8 to 10 minutes. Cool cookies on wire racks.