- In a bowl, whisk together the egg yolk, beer, mustard and vinegar. Drizzle in the oil, whisking constantly. Season with salt and pepper, then set aside.
- With the rack in the middle position, preheat the oven to 400°F (200°C). Line a baking sheet with parchment paper.
- Place the potatoes and sausages on the prepared baking sheet and add half of the oil (1 tbsp/15 ml). Toss to coat and season with salt and pepper. Bake for 25 minutes, stirring halfway through cooking.
- In a bowl, toss the asparagus with the remaining oil and season with salt and pepper. Add to the baking sheet and continue cooking for 8 minutes or until the asparagus is al dente. Slice the sausages into 3/4-inch (2 cm) pieces.
- Meanwhile, in a pot, bring the water and salt to a boil. Add the lentils and onion. Simmer over medium heat for 15 minutes or until the lentils are al dente. Drain, return to the pot and stir in 1/4 cup (60 ml) of the dressing.
- Transfer the lentils to a serving platter. Top with the potatoes, asparagus and sausage slices. Drizzle with the remaining dressing, as desired.