The phrase 'onto which' is correct and used in written English.
It is used to show the action of placing something onto a surface or object. For example: He climbed the ladder onto which he placed the box.
Onto which he would carefully print "60th".
The fronds are tough plastic, onto which Mr. Hiller precisely cuts half-inch-wide leaves.
Some have tablecloths onto which servers place dishes like seared skate with brown butter.
They are projections only, onto which we, in turn, project our own fantasies.
Onto which he would Magic Marker "mental" in front of hospital.
Lauder also bought the ad on the back cover onto which the catalogs are pasted.
They simply use modern off-the-shelf parts, onto which the game's files are loaded.
The story, in any case, is a rickety scaffolding onto which Mr. McG.
This is a homely little cottage of a show, onto which Kitson keeps building needless extensions.
The Wine Cork Trivet is an eight-prong wheel, onto which you screw some corks.
The paintings are screens onto which the viewer can project their own feelings and ideas".
