Here are a few ways I would say it: This food is bland. This food has no flavor. This food is flavorless. This food is tasteless. This food tastes like crap. You can even get a little fancier and say: This dish is so bland and boring that it makes dry oatmeal exciting.

You could say original or maybe "traditional" flavor. It really depends on context or the type of food. If someone orders pizza with no added toppings, they would say "plain" or maybe "cheese". If it was a meal that came from another country English speakers might say "traditional" or "authentic". Sorry I can't give you a more direct answer than to tell you it depends on the food and situation.

I think the expression you are looking for is this: "This food has no added flavour", if talking about a dish in a restaurant I´d rather say "This meal..." As a description in a menu: "A tasteful meal without added flavour" When you say no flavour has been added, it´s understood that the only flavour comes from the main ingredients.

This food is tasteless! Have some ice cream instead!

just say in the simpliest way "this food dont have any flavor' cowboy...