I just started reading Leviathan Wakes, and this is actually covered very early in the novel, before the low quality of the imitation food is even brought up. (It comes before Miller returns to his apartment for the first time, which is the first time he mentions the quality of the food he eats.)
In chapter 2, Miller reflects on all the trade goods that are transported through Ceres:
Platinum, iron and titanium from the Belt. Water from Saturn,
vegetables and beef from the big mirror-fed greenhouses on Ganymede
and Europa, organics from Earth and Mars. Power cells from Io,
Helium-3 from the refineries on Rhea and Iapetus.
So they do grow vegetables and raise livestock for food among the outer planets, and these food products are trafficked through the Belt. I think we can assume that the reason this industry is centered on Ganymede and Europa is those moons' sufficient supply of water - crops grown in the Belt would be only the most water-efficient ones, and livestock raising uses huge amounts of water compared to food crops.
When Miller meets Captain Shaddid, coffee is mentioned specifically:
Real cloth tapestries hung from the walls, and the scent of coffee and
cinnamon came from an insert in her air filter that cost about a tenth
of what the real foodstuffs would have.
So coffee, for one, is available on Ceres, but it's certainly very expensive, at least compared to its cost on 21st century Earth. I think we can assume that other natural food products (aside from the ubiquitous rice, and reconstituted yeast, fungus and vegetable protein) are not generally available on Miller's salary. But ten times the cost of everyday necessities is not an unattainable luxury, so he may treat himself to expensive "real" food on special occasions.
I'll update this answer if I notice any mention later of Miller's taste for real food, but I think these early points cover the crux of the question.