Preheat your oven to the lowest temperature setting and place a baking sheet inside (this will keep your toasts warm!)
In a large shallow bowl, whisk together the eggs, milk, vanilla, and cinnamon until smooth.
In a large skillet over medium heat, warm the butter or oil (about 1-2 tbsp per batch).
While the pan heats up, place two slices of bread (or however many you can fit into your pan!) into the mixture and let sit for about 30 seconds on each side (or until well soaked through).
Once the butter is melted and hot, add in the soaked bread and cook until golden, about 3-5 minutes per side. Transfer to the baking sheet in the oven. Carefully wipe out the skillet and repeat with remaining bread.
Add berries and sugar (if using) to a bowl and mash them up slightly. Set aside.
In a large bowl, add the whipped cream and use an electric beater to beat on high until stiff peaks form (about 3-5 minutes). Add in the yogurt and sugar and beat until combined and fluffy (just about 30 seconds).
Serve the warm french toast topped with whipped yogurt, berries, maple syrup, and walnuts (if using).