Make the pickled mushrooms by combining all the ingredients in a medium sauce pot. Bring to a simmer, turn off heat and let cool. Transfer to a jar and keep refrigerated for up to 2 weeks.
Make the basil pesto ricotta spread by combining the basil, parmesan, olive oil, garlic, salt and pepper in a food processor. Process until finely chopped. Add the ricotta and process again until incorporated.
To assemble, spread the basil pesto ricotta on a slice of toasted bread. Top with a spoonful of the mushrooms and garnish with some microgreens. Serve immediately.