Remove the bottom end and stalks of the fennel bulb.
Reserve the fennel fronds for garnish and the stalks for homemade stock. Thinly slice the fennel bulb. Peel the red onion and julienne it.
Heat a large skillet over medium heat and melt the butter.
Add the slices of fennel and onion and saute for 15 minutes or until tender. Add the balsamic and sugar and cook for an additional 10 minutes, stirring often.
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