Preheat oven to 450.
Cut potatoes into 1 in pieces; toss with 1 tsp oil, season with salt and pepper.
Cut baguette into 1in pieces. Thin slice the onion. Toss fontina with cornstarch.
Arrange potatoes onto baking sheet, bake ~20-25 min, tossing halfway though. Push potatoes to the side, add bread cubes and toast ~5-7min.
Assemble the charcuterie board in the meantime.
Melt 2 Tbsp butter in medium saucepan over med-low heat. Add onions and a pinch of salt and pepper. Cook, stirring occasionally until golden brown, ~12-15min. Set aside 1 Tbsp for later.
To saucepan with remaining onions, add cheese, broth concentrate, and 1c evaporated milk. Cook over low heat, whisking constantly, until melted, bubbly, and thickened, ~2-3min. If too thick, thin with milk.
Garnish fondue with reserved onions, and serve.