Cycling Brussels to Genval

I have done in the past, but not normally by a very direct route.
If you're looking for a short commuting route, I can't help.
But if you want a route for leisure, presumably avoiding traffic, I'd suggest heading for the Bois de la Cambre, going down through there and through the Foret de Soignes, coming out somewhere to the south-west of Hoeilaart. Then take country roads to La Hulpe and on to Genval. The area around the last part is a bit hilly, from memory.