In a pot, melt the butter over medium heat. Once hot, add the onion, garlic, thyme, sage, and a generous pinch of salt. Sauté on low for about 5 minutes, or until the garlic starts to soften.

Add the cornstarch, mix, then pour in the broth and add the Parmesan rind. Simmer for 15 minutes.

Use an immersion blender or a regular blender to blend the soup until smooth. Bring the soup to a simmer once more and turn off the heat.

In a large bowl or measuring cup, beat 3 eggs. Slowly drizzle in the hot soup while continuously whisking.

Drizzle in 2–3 cups of broth before adding the egg mixture back to the pot. Turn the heat back on low-medium and bring the soup back to a simmer.

Preheat the oven to 425℉.

Slice your baguette into ⅓-inch pieces on a bias. Arrange them in a single layer on a sheet pan and lightly brush both sides with olive oil. Evenly sprinkle Parmesan cheese over each slice.

Bake the crostinis for 10–12 minutes, until the cheese is golden brown and the bread is crispy. If the cheese isn't browned enough, place them under the broiler for about a minute to crisp up the top.