Step 1Combine cream, half-and-half and lavender blossoms and leaves in heavy saucepan over medium-high heat until warm. Let stand 30 minutes to allow lavender to steep.
Step 2Whisk together egg yolks and sugar until pale yellow and mixture forms a ribbon when tipped from a spoon. Whisk small amount of warm cream into yolks, then return to saucepan. Cook and stir over medium heat until mixture thickens and coats back of a spoon, about 5 minutes.
Step 3Strain through fine sieve and discard blossom residue. Cover mixture and chill thoroughly, about 1 hour.
Step 4Freeze in ice cream maker according to manufacturer's instructions, about 25 minutes. Garnish with lavender blossoms when serving.