Four days after the Buddha attained enlightenment the leaders of a merchants caravan, Tapussa and Bhallika, happened to see him as he sat under the Bodhi Tree. They stopped, dug into their rations, prepared a dish of mantha and madhupindika, and offered it to him (Vin.I,4). One of the most important and most well-known dishes the Buddha ate was the milk rice offered to him by Sujata just before enlightenment (Khuddaka Nikaya, Buddhavamsa 2.63, 20.16, 25.18). The day before the Buddha passed into final Nibbana he said that the two most blessed alms-giving one could make were one made after which someone attains enlightenment and the one after which a Tathagata attains final Nibbana (D.II,136). Not too much is written about the milk rice offered to the Buddha in the Pali Canon (except what is referenced here), nor the final meal (and there is some controversy over what that final meal actually was), but for the meal just after enlightenment, we have more detailed information. It was a barley meal and honey ball dish known as mantha and madhupindika.
To make mantha first you needed sattu which was parched or roasted barley or rice flour. The grain was roughly ground and then cooked. Sattu was commonly taken by people when they went on a journey; it enabled a filling meal to be prepared with only a little cooking (Ja.III,343; Vin.IV,80). Sattu was mixed with water or sometimes milk and then boiled, proberly with a pinch of salt, and this produced a thick gruel or porridge. Manthu could be eaten together with something else; in the case of the Buddha, with madhupindika. Madhu means honey or more broadly, ‘sweet;’ pinda means a ball or a lump. So madhupindika could refer to a lump of honeycomb but this seems unlikely in this case given that this food would be awkward to carry. I think it is much more likely that madhupindika was the same as a food called laddu in Bihar and ata laddu in UP. Both sweets are made out of cooked grains or pulses soaked in sugar syrup and made into balls. Alternatively, madhupindika may have been the same as what is called gulab jamu throughout northern India. This ball-shaped sweet is made from dough and milk solids fried in ghee and then soaked in syrup. Ananda described madhupindika as having ‘a sweet delicious flavor’ (M.I,114) and anyone who has ever eaten any type of laddu or gulab jamu would agree that this is a pretty good description of these sweets.
According to the Jataka commentary after Tapussa and Bhallika made their offering to the Buddha he gave them a few strands of his hair. When the Chinese pilgrim Husian Tsiang was in a town near Balkh in northern Afghanistan he saw a stupa which supposedly enshrined these hairs. A slightly later legend says that the two merchants were Sinhalese and enshrined the hairs they were given in the beautiful little stupa at Tiriyaya on the north-east coast of Sri Lanka. The most recent of these legends, from Burma, says that Tapussa and Bhallika were from that country and the two hairs the Buddha gave them were enshrined in the Shwedagon Pagoda in Rangoon. If I recall correctly, the Shwedagon Pagoda is not mentioned in any records until the 13th century and there is no mention of its Tapussa and Bhallika associations until the 15th century. As with most legends there are many versions of this one and you can take your pick. The Dhamma, the Truth, is different. It is one (ekham hi saccam ne dutiyam atthi, Sn. 884) and its best if we always be clear about the distinctions between the two.
Other enlightened ones have also ate simple vegetarian foods, like the one mentioned above by the Buddha. The arahant Gosala ate porridge with honey and sugar and then shortly thereafter attained enlightenment (Th. 23).