In PCL:

using Xamarin.Forms;

namespace Project.UI.Controls
{
    public class ExtendedFrame : Frame
    {
        public RoundedFrame()
        {
            HasShadow = false;
            IsClippedToBounds = false;
        }

        public static readonly BindableProperty CornerRadiusProperty = BindableProperty.Create(nameof(CornerRadius), typeof(double), typeof(ExtendedFrame), default(double));

        public double CornerRadius
        {
            get { return (double) GetValue(CornerRadiusProperty); }
            set { SetValue(CornerRadiusProperty, value);}
        }
    }
}

in Android:

using Android.Graphics.Drawables;
using Intouch.Controls;
using Intouch.Droid.Renderers;
using Xamarin.Forms;
using Xamarin.Forms.Platform.Android;
using Color = Android.Graphics.Color;

[assembly: ExportRenderer(typeof(ExtendedFrame), typeof(ExtendedFrameRenderer))]
namespace Project.Droid.Renderers
{
    public class ExtendedFrameRenderer : FrameRenderer
    {
        protected override void OnElementChanged(ElementChangedEventArgs<Frame> e)
        {

            base.OnElementChanged(e);
            if (e.OldElement != null) return;
            var frame = (ExtendedFrame)e.NewElement;
            var vg = ViewGroup;
#pragma warning disable 618
            vg.SetBackgroundDrawable(GenerateShape(frame.BackgroundColor.ToAndroid(), frame.OutlineColor.ToAndroid(),1, frame.CornerRadius, frame.Width, frame.Height));
#pragma warning restore 618
        }

        public static Drawable GenerateShape(Color color,  Color colorStroke, int stokeSize, float strokeRadius, double width, double height)
        {

            var density = Forms.Context.Resources.DisplayMetrics.Density;
            stokeSize = (int) (stokeSize * density);
            strokeRadius = strokeRadius * density;

            var drawable = new GradientDrawable(GradientDrawable.Orientation.BottomTop, new int[] { color, color });
            drawable.SetStroke(stokeSize, colorStroke);
            drawable.SetSize((int)(width*density), (int)(height*density));
            drawable.SetCornerRadius(strokeRadius);

            return drawable;
        }
    }
}



in iOS:

using System.ComponentModel;
using Intouch.Controls;
using Intouch.iOS.Renderers;
using Xamarin.Forms;
using Xamarin.Forms.Platform.iOS;

[assembly: ExportRenderer(typeof(ExtendedFrame), typeof(ExtendedFrameRenderer))]
namespace Project.iOS.Renderers
{
    class ExtendedFrameRenderer : FrameRenderer
    {
        private ExtendedFrame extendedFrame;

        protected override void OnElementChanged(ElementChangedEventArgs<Frame> e)
        {
            base.OnElementChanged(e);

                extendedFrame = (e.NewElement as ExtendedFrame);
                SetCornerRadius();

        }

        protected override void OnElementPropertyChanged(object sender, PropertyChangedEventArgs e)
        {
            base.OnElementPropertyChanged(sender, e);

            if (e.PropertyName == "CornerRadius")
            {
                SetCornerRadius();
            }
        }

        private void SetCornerRadius()
        {
            if (this.NativeView != null && extendedFrame != null)
            {
                this.NativeView.Layer.CornerRadius = extendedFrame.CornerRadius;
            }
        }
    }
}