I'm not really confident about what qualifies as "common food" or "typical western diet", nor the accuracy of the following sources, but I feel like if someone's going to answer OP, they should have something to back it up.

Onions - 5,500 years ago
http://www.vegetablefacts.net/vegetable-history/history-of-onions/

There are two schools of thoughts regarding the home of onion cultivation, and both look at the period 5,500 years ago in Asia. Some scientists believe that onion was first domesticated in central Asia and others in Middle East by Babylonian culture in Iran and West Pakistan

Sugar - 6,000 years ago
https://en.wikipedia.org/wiki/History_of_sugar

The extraction of sugar cane juice from the sugarcane plant, and the subsequent domestication of the plant in tropical India and Southeast Asia sometime around 4,000 BC.

Beans - 7,000 years ago
https://cablevey.com/history-of-dried-beans-how-it-all-started/

Examining the origins of the dry bean takes us back to South America. These, serving as a dietary cornerstone, were initially cultivated over 7,000 years ago in the southern regions of Mexico and Peru.

Corn - 10,000 years ago
https://cropcareequipment.com/blog/corn-farming-history/

People have been farming corn, or maize, for thousands of years. Native civilizations in present-day Mexico first domesticated corn around 10,000 years ago.

Potatoes - 13,000 years ago
https://spudsmart.com/domestication-of-the-potato/

Wild potatoes from the (then) humid coastal plains of South America were probably first eaten by people as early as 13,000 years ago.

Onions.

In fact it's the oldest food, found everywhere domesticated and nobody knows where they come from or who first domesticated them.

Water. We've been drinking it for a billion years.

Bread and beer. The reason that modern civilization exists. Of course, the modern versions are quite different from the ancient ones

Some type of fish or herb, I would imagine. Crops only go back to the advent of agriculture, and lots of the game our ancestors ate isn’t at a grocery store.

Bread and sugar. Everything is bread and sugar.

Depends. In one way, nothing is old, because we shaped and changed everything. But I guess the foods that humans ate at the beginning of our journey is mostly berries and roots.

I'm going to suggest food items that we still take from nature and eat with minimal preparation:

• Honey
• Fish like salmon, trout, grouper
• Shellfish (eg oysters)

We have evidence of shellfish and fish being eaten for a very long time - at least the middle stone age at 140kya - in middens which are 10s of thousands of years old.

Honey is likely to have been a food source - a treat even - even before humans left Africa (so before 100kya) but sadly this would be invisible in the archeological record

I'd imagine a berry of some sort. There could be a berry we still eat that pre-humans also ate. Wouldn't surprise me.

Probably bread

Apples?

Approximately 750,000 years ago: early Paleolithic food gatherers in (modern) Kazakhstan, central Asia, discovered sour crab apples growing wild in the forest.

https://historicsites.nc.gov/all-sites/horne-creek-farm/southern-heritage-apple-orchard/apple-history/origins-apples

Chickens were domesticated around 8000 years ago.

Figs are a very old food, there is archaeological evidence of cultivation of figs at least 11,000 years ago. So meaning not foraged but cultivated on purpose.

In my parts of the world, meat and eggs definitely are the way to go. Nothing grows but grass. Definitely Mountain Goat 🐐 and Highland Cow 🐄 and animals like those. Maybe some herbs to go with that, but crops - nah, not up here.

probably salt

Sugar....