Heat the oil in a large Dutch oven over medium-high heat. Add the onions and sausage. Cook stirring occasionally, until the onions are softened and sausage are browned, about 5 minutes. Add garlic and sauté for a minute or so. Add 1/2 cup water to the pot and once the water is simmering, add the 2 tsp. stock concentrate. Stir until lumps are dissolved. Add greens (you may have to pack them in). Season generously with salt and pepper and pinch of crushed red pepper. Toss the greens well with tongs, and continue simmering on low heat, until the greens are completely cooked and nearly all of the liquid has evaporated, about 10 minutes more.
Taste and adjust seasoning. Transfer to a serving bowl and serve warm.