I like to cook from scratch and I like to make my tomato-based pasta sauce (
marinara) by cooking fresh tomatoes with herbs etc. However, the cooked tomatoes are always too runny for my taste and lacking the thickness that jarred supermarket alternatives do add to the pasta.
I have tried cooking the sauce longer to get more water to evaporate but never was able to reach the "processed" level of consistency/thickness. I am familiar with other forms of thickeners, such as roux or corn starch, but I'd rather not add starch. Is there another thickening agent I could use in my home-cooked marinara sauce?
Fresh tomatoes are insanely watery, so you're starting at a pretty big disadvantage here. Trying to fix it with a thickening agent alone might not be the best plan.
That said, if you want a short answer: use tomato paste, whether homemade or storebought. It'll thicken and improve the flavor. Watery tomato sauce usually has watery flavor, not just watery texture, so any way you thicken besides tomato paste is likely to leave you with a thick sauce that still lacks flavor somewhat.
First off, try to make sure you're using more pasty less juicy tomatoes. It's hard to give very specific advice since this depends a lot on exactly which varieties you get. But everyday supermaket tomatoes are generally too juicy, with the plum tomatoes often (but not always) being somewhat better.
From there, the most common options leave something to be desired:
So the best option, if you're looking for a totally fresh tomato sauce might be something like:
Then since that's all a bit time-consuming, you can save time by buying paste (it's cheap and easy). If you don't have a food mill, peeling and seeding is kind of a pain if you're making larger quantities, so you might consider just chopping reasonably finely and tolerating the skin and seeds.
There's a much longer writeup, "How to Make the Best Tomato Sauce From Fresh Tomatoes", of essentially this technique with an awful lot more attention to detail at Serious Eats. I personally find the lazier versions to be a better balance of time spent and resulting sauce quality, but you might think it's worth it.
Jefromi has already given a thorough answer, but I'd like to offer some advice about reducing the sauce.
Any time you are trying to get rid of liquid, you want as much surface area as possible. As suggested in Jefromi's answer, you can reduce the tomatoes in the oven on shallow baking trays (and if you're doing a larger quantity this is definitely the way to go), but if you are doing it on the stove top, use your widest frying pan, not a sauce pan or stock pot with tall sides. You will need to stir to make sure the parts on the bottom are not sticking and burning (baking tends to avoid this for the most part by giving a much more even heat, although the pan will generally get hotter than the contents).
Reduce the sauce before you season it. Cooking for a long time to reduce the liquid will be hard on most herbs and spices you may use. Also, if the flavor is balanced when the sauce is watery, it will no longer be when it is reduced. I would start with gently frying any onions (and garlic) until translucent then add the tomatoes and reduce it from there. If you are including meat or if you include other vegetables that you don't mind being cooked to unrecognizable mush, things like bell peppers or mushrooms can join in the reducing stage, too, just add them with the tomatoes. But get it to the desired consistency before seasoning it.
Grated Parmesan or Romano helps thicken it . I grow and can my own tomatoes.