In a medium saucepan, heat 1 tablespoon of the olive oil; add the onion half, carrot, and celery rib, and cook, stirring, until golden brown. Add the lentils, chicken stock, bay leaves and thyme and bring to a boil. Reduce the heat to medium-low and simmer until lentils are just tender yet still firm, 15 to 20 minutes. Drain and discard the cooking liquid, bay leaves, thyme stems and vegetables. Place the lentils in a large heatproof mixing bowl and set aside to cool slightly. Add the shallots, garlic, diced celery, lemon juice and zest, tomatoes and chopped herbs and toss the lentils with 1/4 cup of the remaining olive oil. Season with salt and pepper, to taste, and set aside while you prepare the sausage.
In a medium saute pan with the remaining tablespoon of olive oil, or in a grill pan or over a hot grill, cook the sausage until cooked through and browned on all sides. Slice the sausage links thinly on the bias and serve fanned out on small plates with the warm lentil salad spooned over the top.
Garnish with parsley and serve immediately.