How to create thick, hearty garlic marinara sauce?

Ditch the cream and onions, and don't use tomato paste. Take a whole bulb of garlic, peel the cloves and leave them whole.

Heat a cup of good olive oil over medium heat and add the garlic, stirring occasionally until very lightly brown and blistered: be careful not to burn it!

Then add 4 28oz cans of chopped/crushed Italian tomatoes and some chilli, being careful of bubbling oil, and simmer very gently for 4 hours. Season well.

This method produces a thick, intense sauce that you can use for pretty much anything. Keep in the fridge for a week or the freezer for months.

I have always used crushed tomatoes (from a can) in addition to tomato paste. It comes out fairly chunky, but I can still ladle it and pasta will hold it.

I always use the whole tomatoes in the can, crushing each just a bit by hand as I dump them into the pot. It always cooks up very thick and hearty, and the occasional small chunks of tomato that remain are soft enough to be almost sauce consistency, yet add wonderful, tangy flavor bombs throughout the meal.

Use those and the whole garlic cloves that @ElendilTheTall recommends in his/her excellent answer, and you won't find out how delicious it is after having been frozen and reheated, because you won't have any leftovers!

I've always had great results with Hunts canned tomato products, and America's Test Kitchen confirmed this when its experts and novices all picked Hunts when they blind-tested six brands at various price points.

I won't recommend for or against on the cream, as I can't stand the taste of dairy products once they've been heated up, so go with your preferences and/or the experts in here as to whether you keep using it as you fine-tune your recipe.

The key to my garlic sauce is to mince/or run through a garlic press, A LOT of garlic. I use anywhere from 8-10 cloves. Heat 1/4 to 1/3 cups of olive oil over LOW heat in a small saucepan. Add the garlic and slowly poach the garlic in the warm oil for 30 minutes. Watch to make sure the garlic isn't browning too much. Season with a bit salt and add the roasted garlic and oil to whatever pot you use for sauce. Add one 28 oz can of good whole or crushed tomatoes (depending on what consistency you want) and a can of diced tomatoes. Season with salt and pepper and italian seasoning. Simmer slowly for 30-45 minutes, longer is you want a thicker sauce, taste and adjust seasoning. Depending on the quality of the tomatoes a few pinches of sugar may be needed at the end to round out an acidity. There is never any leftovers and it is my go-to for any tomato based pasta sauce, I don't even buy jarred sauce anymore.