Mathf.lerp in an onClick method

I would suggest checking the docs regarding the Marf.Lerp method: https://docs.unity3d.com/ScriptReference/Mathf.Lerp.html

But in any case, here's an example of an Image fade in method using a Coroutine:

 using System.Collections;
 using UnityEngine;
 using UnityEngine.UI;
 
 public class ImageFader : MonoBehaviour
 {
     [SerializeField]
     private Image image = null;
 
     [SerializeField]
     private float speed = 1f;
 
     private void Start()
     {
         StartCoroutine(FadeIn());
     }
 
     private IEnumerator FadeIn()
     {
         SetOpacity(0f);
 
         while (image.color.a < 0.9f)
         {
             SetOpacity(Mathf.Lerp(
                 image.color.a,
                 1f,
                 Time.deltaTime * speed));
 
             yield return new WaitForEndOfFrame();
         }
 
         while (image.color.a < 1f)
         {
             SetOpacity(Mathf.MoveTowards(
                 image.color.a,
                 1f,
                 Time.deltaTime / 5f * speed));
 
             yield return new WaitForEndOfFrame();
         }
 
         Debug.Log("DONE");
     }
 
     private void SetOpacity(float opacity)
     {
         image.color = new Color(
             image.color.r,
             image.color.g,
             image.color.b,
             opacity);
     }
 }

Here's a reference regarding Coroutines: https://docs.unity3d.com/Manual/Coroutines.html