Coarsely chop fontina. Cut mushrooms into 1/4" slices. Coarsely chop spinach. Slice tomato into six 1/4" rounds. Place tomato slices between two paper towels and gently press. This will help absorb excess moisture. Trim ends from baguette and cut into six slices on a slight angle. Combine liquid egg, 3/4 the fontina (reserve remaining to top frittata), half the cream, 1/4 tsp. salt, and a pinch of pepper in a mixing bowl

Heat 2 tsp. olive oil in a medium oven-safe non-stick pan (or cast iron skillet) over medium-high heat. Place mushrooms in hot pan and stir occasionally until lightly browned, 4-5 minutes. Add spinach to pan and cook until wilted, 30 seconds to 1 minute. Remove from burner. Reserve pan; no need to wipe clean. Allow vegetables to cool enough to handle.

Transfer vegetables to a clean towel and press to absorb any liquid. Reserve pan; no need to wipe clean.

Place vegetables and egg mixture in pan used to cook vegetables. Stir to evenly distribute ingredients. Lay tomato slices in mixture and sprinkle remaining fontina over assembled frittata.

Place frittata in oven and bake until egg is set and cheese is bubbly, 15-18 minutes. Place bread slices on prepared baking sheet. Drizzle with 1/2 tsp. olive oil and season with a pinch of salt and pepper. Once frittata has baked 8 minutes, put bread in oven and toast until lightly browned, 5-7 minutes.

Carefully remove frittata from oven and place on a trivet. Wrap a towel around handle to indicate pan handle is hot. Garnish finished frittata with basil pesto. Serve frittata from pan alongside crostini.