Peel the Jerusalem artichoke and cook in a saucepan with water until soft.
Mix the flour, milk, baking powder and egg yolk with a mixer to a smooth dough. Let the dough rest for 30 minutes, then season with rosemary and pepper.
Cut the Jerusalem artichoke into very small cubes or mash it with a fork (depending on how well the baby is already eating) and add to the batter. Beat the egg white until stiff and fold into the dough.
Heat a pan with a little rapeseed oil. Portion the dough in the pan with a spoon. Bake the blini until golden brown on both sides. Replenish oil as needed.
Serve immediately, otherwise the blini will collapse.