In a large bowl, stir together the flour, sugar, eggs, and melted butter.
Add ½ cup (120 ml) of milk at a time, stirring vigorously, making sure that each addition is completely incorporated and that the batter is smooth before adding more milk.
Strain the batter through a fine mesh sieve into another large bowl. The batter should be very liquidy and have no lumps. Rest in the fridge for at least 30 minutes, up to overnight.
In a medium nonstick pan over medium heat, pour ⅓ cup (80 ml) of the batter in the center, and swirl the batter around to reach the edges of the pan.
Cook the crêpe until bubbles rise to the surface and the bottom is golden brown, about 3 minutes. Flip and cook on the other side until the edges start to crisp slightly, about 1 minute more.
Transfer the crêpe to a wire rack to cool. Repeat with the remaining batter.
Cut the crêpes in half. Spread chocolate hazelnut spread or whipped cream evenly over each crepe half, then place a slice of banana or strawberry half on the right edge. Fold up the bottom of the crêpe, then fold toward the left side to make triangular shape. Repeat with the remaining ingredients.