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.