The trick is to reduce the tomato well before adding the broth. When you blend fresh tomatoes and let them cook over medium-high heat for about ten minutes, the flavor concentrates and they lose that watery character they have at first. Then you add the hot broth (chicken or vegetable works well), the noodles, and a bit of salt, but the important thing is that tomato purée has been simmering on its own, without any dilution. If you want it even thicker, you can save one tomato without blending it and grate it raw at the end, or just let the soup reduce a bit more after the noodles are done.