MELT butter in large skillet. Add walnuts, onions and garlic; cook 2 to 3 min. or until walnuts are lightly toasted and onions are tender, stirring occasionally. Set aside.
ADD cauliflower to small amount of boiling water in medium saucepan; cover. Bring to boil. Reduce heat to medium-low; simmer 8 to 10 min. or until cauliflower is crisp-tender. Drain.
PLACE cauliflower in serving dish; top with the walnut mixture. Sprinkle with bacon bits.