Attach sous vide wand to stock pot. Fill pot with water according to manufacturer's instructions.
Preheat sous vide wand to 183°F.
In a small bowl, whisk together stock, lemon juice, lemon zest, mustard, shallots and brown sugar.
Place blueberries, butter and beef stock mixture in a food-safe plastic bag and seal, removing as much air as possible.
Submerge bag in water and set timer for 20 minutes.
At end of cook time, carefully remove bag from water. Open bag and serve sauce over steak.
Nutrition information per serving, (no serving size): 125 Calories; 54.9 Calories from fat; 6.1g Total Fat (3.6 g Saturated Fat; 0.2 g Trans Fat; 0.3 g Polyunsaturated Fat; 1.7 g Monounsaturated Fat;) 15 mg Cholesterol; 243 mg Sodium; 16.6 g Total Carbohydrate; 2.1 g Dietary Fiber; 1.4 g Protein; 0 mg Calcium; 0.4 mg Iron; 154 mg Potassium; 0.1 mg Riboflavin; 0.6 mg NE Niacin; 0.1 mg Vitamin B6; 0 mcg Vitamin B12; 24 mg Phosphorus; 0.2 mg Zinc; 0.6 mcg Selenium; 8.8 mg Choline.