Beet root (usually as juice) is a supplement with a high nitrate content that is said to improve physical performance secondary to nitric oxide. It appears to have some evidence for this claim in healthy athletes.
Beetroot tends to be dosed on the nitrate content, with around 0.1-0.2mmol/kg (6.4-12.8mg/kg) being the target for nitrate. This is about 436mg for a 150lb person, which is comparable to half a kilogram (500g) of the beetroots themselves (wet weight).
Consumption of beetroots for the nitrate content can be either via a puree or smoothie, or the beets themselves can be baked in an oven into chips. The aforementioned cooking techniques do not appear to reduce the nitrate content.