In a medium saucepan, add the lentils and enough water to cover by 2 inches. Add the onion and bay leaf, bring to a boil and cook until tender, about 20 minutes. Discard the bay leaf.
Meanwhile, in a shallow bowl, combine 1/4 cup EVOO and the garlic; let stand for 15 minutes.
In a medium skillet, add the sausages and enough water to reach a depth of 1/4 inch. Drizzle generously with EVOO and bring to a boil, then lower the heat and simmer until the water is evaporated, about 8 minutes. Cook the sausages until slightly crisp, 3 to 4 minutes longer. Transfer to paper towels to drain.
In a large bowl, drizzle the arugula with EVOO and the vinegar; season with salt and pepper.
Toss the hot lentils with the garlic oil and parsley. Serve with the sausages and arugula.