Step 1

In a small saucepan, melt 2 and 1/2 tablespoons butter over medium heat. Whisk in flour and cook for 2 minutes, stirring frequently. Remove from heat and whisk in warm milk. Return to heat and cook, whisking constantly, until thickened, 2 to 3 minutes

Step 2

In a small bowl, whisk yolks. Whisking constantly, dribble yolks into the saucepan until they are completely incorporated. Scrape mixture into a large bowl and cool to room temperature

Step 3

Heat oven to 375 degrees. Grease a 1 and 1/2 quart soufflé dish with remaining 1/2 tablespoon butter and sprinkle the bottom and sides evenly with cheese

Step 4

In a clean mixing bowl, beat egg whites with a pinch of salt until they form stiff peaks, but are not dry. Fold a third of the whites into yolk mixture to lighten it, then fold in remaining whites. Gently fold in vegetables

Step 5

Immediately scrape mixture into prepared dish. Transfer to oven and bake until puffed, golden and slightly firm to the touch, about 30 minutes. Do not open the oven door during baking. Serve immediately