I don't believe they can--I just talked to an appliance salesman about this and he claimed they need several inches of clearance above them (I forget what can and can't be above them, but a countertop was not allowed).

This oven from Verona is sized the same as many 24" undercounter electric ovens; however, the manufacturer states the oven should be mounted in a tall cabinet over 22" above the floor, perhaps because the hot air that escapes from the two air vents at either side of the controls would be at a child's eye height.

If the lack of 208 or 240v power is the issue, there are a few undercounter 120V, 20 amp ovens that are mountable undercounter, including the new Bosch shown here which is available in 27" or 30" widths. These can work as microwaves as well. The GE Advantium 120 (30" only) and one model from Electrolux can also be used undercounter. Only the Bosch has a full-length rack. None of these are as roomy as standard 24" ovens, much less 27" or 30" though, but they may be adequate.

there is NO ul or aga approval to install a gas oven undercounter because of combustion ;the last company to make them was yrs ago-stoves of america-which went bankrupt and the people who bought them have replaced them with electric ovens because of their high service record.BTW the person on here who suggested the verona should know they do not have ul/aga approval to install undercounter and should not do so

I said exactly that about the Verona, that it's not approved for undercounter use, only in a wall cabinet two feet or more above the floor.

I saw one of those Stoves of America ovens years ago in a store - despite the brand name they were British IIRC. Only available for a few years in the US and they didn't seem to sell many.so they're probably hard to find secondhand (although someone has a supposedly unused one on eBay at the moment and one recently sold on craigslist, so they're a few out there). Model# was 850 GRF; they're 30"w. Apparently a high failure rate with the electronics.