Mix together water, sugar and cornstarch. Microwave for a minute and then stir. Continue microwaving and stirring in 30 second intervals until thick.
Meanwhile, mix together the egg yolks, lemon juice and lemon zest. Stir into the filling mixture and microwave for another 30 seconds to a minute until thick. Pour into crust.
Beat egg whites and cream of tartar on high until foamy. Beat in sugar, 1 T at a time. Beat in vanilla. Continue beating until stiff and glossy and stiff peaks form. Do NOT under beat.
Cover pie with meringue all the way to edge of the crust. Using a spatula or knife, pat down and lift up to form peaks.
Bake at 400 degrees for 8-12 minutes just until golden on the edges.