In a small frying pan, melt butter; add onion and saute until golden (5 to 7 minutes).
Set aside.
In large bowl of an electric mixer, stir together 1 cup of the all-purpose flour, 1 cup of the whole wheat flour, sugar, salt, and yeast.
Measure and set aside about 2 tablespoons onion-butter; then mix remaining onion-butter into yeast mixture.
Pour in hot water; beat at low speed for 2 minutes.
Add 1 more cup whole wheat flour and beat at high speed for 2 minutes.
Stir in remaining 1 cup whole wheat flour and enough of the all-purpose flour (about 1 cup) to make a soft dough.
Sprinkle about 1/3 cup of the remaining all-purpose flour on a board; turn dough out and knead until smooth (about 5 minutes), adding flour as needed to prevent sticking.
Turn dough over in a greased bowl; cover and let rise in a warm place until doubled (about 1 hour).
Punch dough down, divide into 20 equal pieces, and roll each into a ball; place balls about 4 inches apart on greased baking sheets.
With greased fingers, flatten each into about a 4-inch circle.
Spread about 1/4 teaspoon reserved onion-butter on top of each circle.
Cover and let rise until almost doubled (about 50 minutes).
Bake in a 375° oven for 20 to 25 minutes or until brown.
Let cool on racks.