In a saucepan, melt the butter over medium heat. Sprinkle with flour and stir. Pour the milk and bring to a boil while whisking. Salt and pepper. Remove from heat.
In another saucepan, heat the oil over medium heat. Cook celery, carrots, onion, and garlic for 2 minutes.
Add the potatoes, broth, sauce, and nutmeg. Salt and pepper. Cover and cook for 8 to 10 minutes, stirring occasionally.
Add the white beans, green peas, and parsley. Stir. Rectify the seasoning as needed.
Pour the mixture into four to six ramekins.
On a lightly floured surface, roll the dough into four to six circles, 5 inches in diameter. Place the circles of dough on the ramekins. Brush the dough with egg yolk.
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